Single photon emission computed tomography system

ABSTRACT

A single photon emission computed tomography system includes a detector assembly adjacent a field of view and a collimating assembly disposed between the detector assembly and the field of view. The collimating assembly includes at least two spaced-apart collimating vanes of photon-attenuating material. The system further includes a photon-blocking member disposed between the field of view and the detector. The blocking member has an aperture defined therethrough. The system further includes a mask disposed adjacent the detector assembly having at least one aperture defined therethrough. A displacement actuator moves the photon-blocking member relative to the detector assembly.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/872,253, filed Jun. 18, 2004, which claims priority to U.S.Provisional Patent Application Ser. No. 60/480,381, filed Jun. 20, 2003.U.S. patent application Ser. No. 10/872,253 is a continuation-in-part ofU.S. patent application Ser. No. 10/358,961, filed Feb. 5, 2003, nowU.S. Pat. No. 7,015,476 which is a continuation-in-part of U.S. patentapplication Ser. No. 09/549,435, filed Apr. 14, 2000, now U.S. Pat. No.6,525,320, which claims priority from U.S. Provisional PatentApplication Ser. No. 60/129,239, filed Apr. 14, 1999 and Ser. No.60/151,378, filed Aug. 30, 1999. This application also claims priorityfrom U.S. Provisional Patent Application Ser. No. 60/480,381, filed Jun.20, 2003, the entire content of all of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to imaging systems and, morespecifically, to a single photon emission computed tomography system.

BACKGROUND OF THE INVENTION

Medical radionuclide imaging (Nuclear Medicine) is a key component ofmodern medical practice. This methodology involves the administration,typically by injection, of tracer amounts of a radioactive substance,which subsequently localizes in the body in a manner dependent on thephysiologic function of the organ system being studied. The radiotraceremissions, most commonly gamma photons, are imaged with a detectoroutside the body, creating a map of the radiotracer distribution withinthe body. When interpreted by an appropriately trained physician, theseimages provide information of great value in the clinical diagnosis andtreatment of disease. Typical applications of this technology includedetection of coronary artery disease (thallium scanning) and detectionof cancerous involvement of bones (bone scanning). The overwhelming bulkof clinical radionuclide imaging is performed using gamma emittingradiotracers and detectors known as “gamma cameras”.

Gamma cameras typically consist of a large scintillation crystal (e.g.sodium iodide) having the property of emitting light when struck bygamma photons. Affixed to the rear of this crystal are multiplephotomultiplier tubes with associated circuitry to detect the lightflashes and to locate their position within the scintillation crystal.In front of the crystal is a collimator, typically consisting of severalmillimeters of lead with multiple holes penetrating it. The collimatorserves to absorb all incoming photons except those approaching thecrystal generally from the appropriate direction. The crystal,photomultiplier tubes and associated circuitry are typically enclosed ina large lead case that serves to shield the detector from unwantedexternal radiation. The entire apparatus is mounted on a gantry with amotorized apparatus for positioning the detector near the patient.

A gamma camera provides a two-dimensional image of radiotracerdistribution. However, the distribution of radiotracers within the bodyis typically three-dimensional. The technique of single photon emissiontomography (SPECT) is used to create three-dimensional, tomographicimages similar to a “radionuclide CT scan” by using computer processingto “reconstruct” the three-dimensional tracer distribution from a seriesof two-dimensional gamma camera images obtained from multiple anglesaround the patient. This is almost universally accomplished by mountingone or more gamma cameras to a motorized gantry and orbiting them aroundthe patient. The data thus acquired is then processed to yield thethree-dimensional images.

The three-dimensional SPECT images have been demonstrated to providehigher image contrast and to reduce apparent overlap of body structures.SPECT imaging is now considered to be the state-of-the-art inradionuclide imaging of the heart and now accounts for more than half ofall cardiac nuclear imaging performed in the United States.

Despite its many advantages, SPECT imaging is not yet available to allpatients who might benefit from it. Current SPECT instrumentation has anumber of disadvantages which have impeded its wider implementation.

Current SPECT systems are bulky, typically requiring a large, dedicatedroom to house them. The collimating systems are relatively inefficient,blocking a high percentage of emitted radiation. Thus, most new clinicalsystems simultaneously utilize two or more gamma camera detectorsmounted on a single gantry. Since each detector typically weighs severalhundred pounds, the supporting gantry must be large and heavy. MostSPECT installations require specially constructed rooms with added floorreinforcement. Since accurate image reconstruction requires precisedetector placement, SPECT systems require heavy positioning systemsconsisting of motors and gearing capable of moving and positioninghundreds of pounds of apparatus to a precision of approximately amillimeter. These systems are necessarily large, heavy and expensive.

Although there is great medical need to image patients in a variety ofsettings, including doctors' offices, emergency rooms and intensive careunits, the great size and bulk of current SPECT systems has requiredthem to be in a fixed location, typically a hospital Radiology orNuclear Medicine department. There are significant medical and patientconvenience advantages to having cardiac SPECT imaging performed in theimmediate presence of the attending Cardiologist. Many studies haveshown that the cost of care delivered in an outpatient office setting isless than that of a hospital setting. Despite these compelling factors,the size and cost constraints of current systems have greatly limitedtheir penetration into the community and have particularly limited theiravailability in physicians' offices. In addition, the large spacerequirements of current systems have imposed significant costs onhospitals providing SPECT services.

Current SPECT systems have additional limitations. As the gamma camerasorbit around the patient, large multi-conductor cables are required tocarry power and data to and from each detector. These cables arerepeatedly flexed during system operation and are a frequent cause ofequipment breakdown.

The large and heavy nature of existing systems has dictated a mechanicalgantry design that is highly stable, yet cost effective. This hasresulted in systems in which the patient must lie in a supine (flat onthe back) position on a narrow platform that extends into a verticallyoriented gantry. In order to permit the detectors to be as close aspossible to the chest and to enable the large, moving detectors tosafely pass around the patient, current systems require the patient tomaintain one or both arms in an uncomfortable position held over thehead. This is painful for most patients and impossible for some. Inaddition, the supine position is uncomfortable for many patients,particularly for those with back problems. Many patients feelclaustrophobic when inside the equipment. The narrow platform requiredto permit camera rotation around the patient is uncomfortable for largeindividuals and is often perceived as insecure or precarious by thoseundergoing scans. Also, the fact that the patient is partially enclosedby the equipment during imaging may serve to limit physician or nursingaccess to critically ill patients.

SUMMARY OF THE INVENTION

The present invention provides a plurality of imaging systems andcomponents therefor. According to one embodiment, a single photonemission computed tomography system includes the detector assemblyadjacent a field of view. The detector assembly includes aphoton-responsive detector operable to detect if a photon strikes thedetector. The detector assembly is operable to detect photons emittedfrom the field of view. A collimating assembly includes at least twospaced apart collimating vanes of photon-attenuating material. Each ofthe vanes is disposed between the detector assembly and the field ofview. A photon-blocking member is disposed between the field of view andthe detector. The blocking member has an aperture defined therethroughfor passage of photons aligned with the aperture. A line of response isdefined from the detector assembly through the aperture. A mask isdisposed adjacent to the detector assembly and between the detectorassembly and the field of view. The mask has at least one aperturedefined therethrough for passages of photons aligned with the aperture.A displacement actuator moves the photon-blocking member relative to thedetector assembly such that the aperture is displaced relative to thedetector assembly and the line of response is swept across at least aportion of the field of view. A longitudinal axis may be defined throughthe field of view. In some embodiments, the photon-blocking member is aring of photon attenuating material generally coaxial with thelongitudinal axis and disposed in an imaging plane. The aperture definedin the photon-blocking member comprises a plurality of spaced-apartaperture slots defined through the ring of photon attenuating material.The aperture slots may be generally parallel to the longitudinal axisand perpendicular to the imaging plane. In some embodiments, thephoton-blocking member is disposed between the collimating assembly andthe field of view.

In some embodiments, the mask is a ring of photon attenuating materialgenerally coaxial with the longitudinal axis and disposed in an imagingplane. The aperture defined in the mask comprises a plurality ofspaced-apart aperture slots defined through the ring of photonattenuating material. The aperture slots may be parallel to thelongitudinal axis and perpendicular to the imaging plane.

In some embodiments, the collimating vanes are each generally ringshaped and disposed in a plane perpendicular to the longitudinal axis.Each vane has a depth extending radially from a front edge to a rearedge in the plane and a thickness perpendicular to the plane. The depthof each vane is greater than the thickness. The collimating assembly maybe constructed such that radiolucent material is disposed in the spacesbetween the vanes.

In certain embodiments, the collimating assembly includes only vanesthat are generally parallel to each other and generally perpendicular tothe longitudinal axis.

In some embodiments of the present invention, the detector assembly isgenerally ring shaped and coaxial with the longitudinal axis. Thedetector assembly may be a ring of scintillation material with anoutwardly adjacent ring of photodetectors. Alternatively, the detectorassembly may be a plurality of solid-state photon detectors disposedgenerally in a ring.

In further embodiments, the detector assembly, the collimating assembly,the photon-blocking member and the mask are each generally arcuatemembers extending between a pair of spaced-apart ends. The arcuatemembers may extend over an arc between 180 and 360 degrees.

In another alternative embodiment, the photon-blocking member and themask are each generally flat members.

In some embodiments, the longitudinal axis is generally vertical and thesystem according to the present invention is disposed generally in ahorizontal imaging plane.

The collimating assembly may be constructed of a stack of alternatingsheets of photon-blocking material and sheets of radiolucent material,the sheets of photo-blocking material defining the collimating vanes.

In some embodiments, the system further includes a chair-like basehaving a generally horizontal bottom portion for supporting a patient'ships and a generally vertical back portion for supporting the patient'sback. The detector assembly, the collimating assembly, thephoton-blocking member and the mask may be generally ring-shaped membersthat are disposed such that the field of view is positioned to receivethe patient's head when the patient is seated in the chair-like base.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of a preferred embodiment of the presentinvention optimized for cardiac SPECT, showing the overall configurationof the system and the positioning of the patient;

FIG. 1B is an additional perspective view of the embodiment of FIG. 1A;

FIG. 2 is a perspective view of one embodiment of an individual detectormodule for detecting photons during SPECT imaging;

FIG. 3 is a perspective view of an aperture arc for an embodiment of thepresent invention that is optimized for cardiac SPECT, with a singleradiation detection module shown behind the arc to demonstrate relativepositioning;

FIG. 4 is a cross-sectional detailed view of a small portion of anaperture arc, showing details of one embodiment of an aperture edgetreatment;

FIG. 5 is a cross-section detailed view similar to FIG. 4, showing analternative embodiment of an edge detail;

FIG. 6 is a detailed view similar to FIGS. 4 and 5, showing yet anotheralternative embodiment of an edge detail;

FIG. 7 is a cross-section detail of a portion of an aperture arc,including adjustable end pieces for providing an aperture with anadjustable width;

FIG. 8 is a perspective view of a portion of an aperture arc and oneadjustable end piece;

FIG. 9A is a diagrammatic top view showing the relative positions of theslotted aperture arc, the arc of detectors and the patientfield-of-view;

FIG. 9B is a diagrammatic top view showing how lines of response of theindividual detectors provide multiple angular projections through thebody;

FIG. 10A is a top schematic view of a single detector module and a smallsection of the aperture arc at a first rotational position of theaperture arc;

FIG. 10B is a view similar to FIG. 10A but with the aperture arc at asecond position;

FIG. 10C is a view similar to FIGS. 10A and 10B but with the aperturearc at a third position;

FIGS. 11A-F are a series of diagrammatic top views of the presentinvention;

FIG. 12 is a partially transparent perspective view of an alternativeembodiment of an imaging section for the present invention, includingaperture arcs and collimator vanes that are angled;

FIG. 13 is a perspective view of a cross-plane (longitudinal)collimation assembly showing its relationship to the detector modules;

FIG. 14 is a view similar to FIG. 13 but including the aperture arc andshowing the lines of response from one detector module;

FIG. 15 is a plot showing the in-plane spatial resolution at differentdepths using the present invention versus a traditional “highresolution” parallel-hole collimator;

FIG. 16 is a cross-sectional view of a portion of a parallel vanecollimator according to the present invention;

FIG. 17 is a perspective view of the support assembly for one embodimentof an imaging arc according to the present invention;

FIG. 18 is a perspective view of a support assembly similar to FIG. 17,with additional tension members;

FIG. 19 is a cross-sectional view of a portion of a parallel vanecollimator and a sensor assembly according to the present invention,showing the relative depth of the collimator vanes;

FIG. 20A is a cross-sectional top view of one embodiment of a moveableaperture arc extension vane;

FIG. 20B is a view similar to FIG. 20A with the vane shown at adifferent position;

FIG. 21 is a perspective view of a portion of a lower support member anda portion of an aperture arc according to one embodiment of the presentinvention;

FIG. 22 is a rear perspective view of a sensor assembly for use with thepresent invention;

FIG. 23 is a front perspective view of the sensor assembly of FIG. 22;

FIG. 23 is a side elevational view the sensor assembly of FIGS. 22-23;

FIG. 25 is a cross-sectional detailed view of a portion of a sensormodule;

FIG. 26 is a front view of an embodiment of a sensor module;

FIG. 27 is a view of a pair of sensor arrays as viewed through thecollimator assembly;

FIG. 28 is a perspective view of a portion of one embodiment of ascintillator-based cylindrical detector module;

FIG. 29A is a perspective view of another embodiment of a detectormodule using a rectangular bar-shaped piece of scintillation material;

FIG. 29B is a side elevational view of the module of FIG. 29A with photodetectors at the top and bottom;

FIG. 29C is a view similar to FIG. 29B but with the photo detectorspositioned at the rear face of the scintillation material;

FIG. 30 is a perspective view of a detector module with a block ofscintillation material with a trapezoidal cross section;

FIG. 31A is a perspective view of a masked detector configuration basedon a rectangular shaped piece of scintillation material;

FIG. 31B is a perspective view of a masked detector configuration basedon a cylindrical shaped piece of scintillation material;

FIG. 31C is a perspective view of a masked detector configuration basedon a piece of scintillation material with a trapezoidal cross section;

FIG. 32 is a perspective view showing construction details of abar-shaped, masked detector module similar to FIG. 31A, but withphoto-detectors placed along its rear face;

FIG. 33 is a diagrammatic representation of the directions of concurrentdetector and aperture arc motion for one embodiment of the invention;

FIG. 34 is a perspective view of a two-dimensional scintillator baseddetector having masking strips according to the present invention;

FIG. 35 is a perspective view of a portion of an aperture arc with acalibration module disposed by the aperture;

FIG. 36 is a top diagrammatic view of yet another embodiment of thepresent invention, which makes use of two-dimensional detectors andlinear blocking members;

FIG. 37 is a perspective view of an imaging arc;

FIG. 38 is a cross-sectional view of one embodiment of an imaging areaccording to the present invention;

FIG. 39 is a cross-sectional of another embodiment of an imaging arcaccording to the present invention;

FIG. 40 is a cross-sectional view of yet another embodiment of animaging arc according to the present invention;

FIG. 41 is a schematic view of a patient field of view along with atraditional gamma camera shown in two different positions;

FIG. 42 is a sinogram representing data received by a traditional gammacamera;

FIG. 43 is another sinogram representing data for a traditional gammacamera under a different set of conditions;

FIG. 44 is a sinogram representing data from an imaging system accordingto the present invention;

FIG. 45 is a schematic similar to FIG. 41 showing the equivalentaperture arc and detector arc positions for the present invention;

FIGS. 46 and 47 are schematics showing the geometry necessary forconversion of positions of the aperture arc and detector arc for usewith traditional two-dimensional data processing;

FIG. 48 is a top view of a portion of an aperture arc with oneembodiment of a radioactive calibration source and holder;

FIG. 49 is a perspective view of the outer shell or holder portion of acalibration source;

FIG. 50 is a perspective view of the holder of FIG. 49 with a photonblocking shield added;

FIG. 51 is a perspective view similar to FIGS. 49 and 50 with theaddition of radiation source retainers;

FIG. 52 is a perspective view similar to FIGS. 49-51 with a cylindricalradiation source added;

FIG. 53 is a perspective view of the calibration source of FIG. 52attached to a portion of an aperture arc;

FIG. 54 is a perspective view of one side of a portion of an aperturearc showing receivers for receiving the calibration source;

FIG. 55 is a perspective view similar to FIG. 54 illustrating thecalibration source being inserted into the holders;

FIG. 56 is a perspective view of an alternative embodiment of acalibration source holder;

FIG. 57 is a perspective exploded view of the holder of FIG. 56;

FIG. 58 is a perspective view of a calibration source for use with aholder of FIGS. 56 and 57;

FIG. 59 is an exploded view of the calibration source of FIG. 58;

FIG. 60 is a perspective view of the holder of FIGS. 56 and 57 with thecalibration source of FIGS. 58 and 59 received therein;

FIG. 61 is a perspective view of the source and holder of FIG. 60showing the opposite side;

FIG. 62 is a perspective view of an upper and lower support memberforming part of an imaging arc along with the holder and source of FIGS.60 and 61;

FIG. 63 is a perspective view similar to FIG. 62 with the addition of aportion of an aperture arc added;

FIG. 64 is a perspective view similar to FIG. 63 with a portion of theupper support member cut away;

FIG. 65 is a detailed perspective view of a bottom part of the aperturearc and source holder of FIG. 64;

FIG. 66 is a perspective view of the calibration source and holder withthe calibration source partially inserted;

FIG. 67 is a side view of the source and holder of FIG. 66 with thesource partially inserted;

FIG. 68 is a side view similar to FIG. 67 with the source fullyinserted;

FIG. 69 is an additional perspective view of the source and holder fromthe opposite side as compared to FIG. 66, with the source partiallyinserted;

FIG. 70 is a perspective view similar to FIG. 69 with the source shownfully inserted;

FIG. 71 is a top view of the calibration source holder of FIG. 70 withexemplary dimensions indicated thereon;

FIG. 72 is a top view of a calibration source;

FIG. 73 is a top view of a calibration source storage container with aplurality of calibration sources disposed therein;

FIG. 74 is a perspective view of a curved crystal used in someembodiments of the present invention;

FIG. 75 is a perspective view of the crystal of FIG. 74 backed by aplurality of photo tubes;

FIG. 76 is a perspective view of a brain imaging version of the presentinvention;

FIG. 77 is a perspective view, with partial cutaways, showing anembodiment of an imaging ring including a mask ring;

FIG. 78 is a cross-sectional view of the imaging ring of FIG. 77 takenalong lines A-A;

FIG. 79 is a cross-sectional view of the imaging ring of FIG. 77 takenalong lines B-B;

FIG. 80 is a detailed view of a portion of an imaging ring or arcillustrating the difficulty in distinguishing between closely spacedlines of response;

FIG. 81 is a detailed view similar to FIG. 80, but further including amask ring or arc and illustrating the potential improved resolution;

FIG. 82 is a detailed view similar to FIG. 81 with the mask ring or arcmoved a short distance, counterclockwise;

FIG. 83 is a detailed view similar to FIG. 81 with the aperture arc orring rotated a short distance counterclockwise;

FIG. 84 is a perspective view of a flat or planar version of an imagingdevice according to the present invention;

FIG. 85 is a perspective view of an arcuate version of an imaging deviceaccording to the present invention; and

FIG. 86 is a perspective view, partially cut away, of an alternativeversion of an imaging ring according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, the preferred embodiment and examples shownshould be considered as exemplars rather than as limitations on thepresent invention.

I. General Overview

One aspect of the present invention comprises a system for performingsingle photon emission computed tomography (SPECT). This system includesa radiation detector assembly consisting of a multiplicity of radiationdetector modules preferably positioned around an arc, typically over 180to 360 degrees. In-plane (axial) collimation is provided by a movablearc or ring extending over an angular range similar to that of theradiation detector assembly (typically 180-360 degrees). Cross-plane(longitudinal) collimation is provided by a plurality of vanes or sheetsof photon-attenuating material held in a stationary position andoriented parallel to the transaxial plane (perpendicular to thelongitudinal axis). Optionally, these vanes may be separated by sheetsof a radiolucent spacer material such as Styrofoam® or other plastic.Some embodiments of the present invention also include a patient chairor support structure.

II. Discussion of Chair, Arc Configuration, and Patient Positioning

FIGS. 1A and 1B illustrate a preferred embodiment of the presentinvention optimized for cardiac SPECT, showing the overall configurationof the system 100 and the positioning of the patient 102. The opening104 for patient entry and egress is shown. The imaging section 106 ofthe system extends as an arc over the right side of the patient's chest.The imaging section consists of a lead shielded housing with internalcomponents as described below. The imaging section is supported by astand 108 affixed to a base 110. Together, the rear portion of theimaging section and the stand form the “back” of the patient support.The patient is seated upon an adjustable seat 112. The vertical heightof this seat may be adjusted so as to position the patient's heartwithin the appropriate portion of the imaging device. Such adjustmentmay be performed by means of electrical motors, hydraulic devices orother means. The seat is optionally adjustable so as to swivelhorizontally, thus easing patient entry and egress from the seatedposition. The stand and base may also include or support the electronicsnecessary for processing scans, as well as any necessary controls ordisplays.

As shown, unlike in the prior art systems, the patient is seatedgenerally upright so that their torso is generally vertical. The lighterweight, simpler design, and reduced bulk of the present system cooperateto allow this positioning. For definitional purposes, the areasurrounded by the imaging section 106 will be referred to as a field ofview. Also for definitional purposes, it may be said that a longitudinalaxis, generally aligned with the longitudinal axis of the patient'storso, extends through the field of view. It may be said that thelongitudinal axis is generally vertical to distinguish the positioningof the present system from the typical systems where the patient isforced into a horizontal position. In actuality, the generally verticallongitudinal axis may be reclined somewhat, as shown, to increasepatient comfort.

As will be clear to those of skill in the art, it is very important toimage the appropriate portion of the human patient, in order to acquiredata about the portion of the patient that is of particular interest.For example, the preferred embodiment of the present invention isdesigned to image the patient's heart. Therefore, it is important thatthe portion or slice being imaged includes the patient's heart. However,the exact position of the patient's heart within their chest is notalways easily determined from an external exam. In prior art systems,the patient is positioned in front of the detector(s) while the operatorviews a low-resolution, two-dimensional display known as apersistence-scope (p-scope). The persistence scope image is necessarilyof low quality due to its need to be continually updated as the patientis repositioned and due to the two-dimensional nature of its images.Operator error in patient positioning is not uncommon and, when itoccurs, results in a useless scan. According to another aspect of thepresent invention, a cardiac scan may be preceded by a “quick scan” ofthe patient's chest so as to properly locate the heart so as to adjustthe position of the chair so that the heart is properly positioned forimaging by the imaging section 106.

The “quick scan” is possible with the present invention for severalreasons, which will become clear after reviewing the entirety of thisspecification. Systems of the present art must partially orbit thepatient in order to acquire three-dimensional imaging. Movement of thelarge, heavy (typically 450-500 pound) detectors must be started andstopped within seconds if rapid three-dimensional positioning images areto be obtained. This is both mechanically difficult and may present ahazard to the patient from the rapid movement of large and heavydetectors. The present invention requires the movement of only anaperture are to image the portion of the patient in the field of view ofthe imaging section 106. The aperture arc is preferably hidden from thepatient inside a housing, and can be moved much more quickly and safelythan can prior art gamma camera. Also, a full scan requires the arc tomove only a short distance, unlike a gamma camera where the camera hasto move a long distance. In addition, the present invention acquiresimage data more quickly than prior art devices. Therefore, a fast, lowcount, three-dimensional image may be acquired by quickly moving theaperture arc within the housing. This low count image may bereconstructed almost instantly with state-of-the-art computers anddisplayed immediately as slices, or preferably, as rotating surfacerendered or maximum-intensity-projection images. Such volume-renderedimages clearly reveal the underlying patient anatomy and may be used toreliably determine the position of the heart prior to the start ofroutine, high count imaging.

In embodiments of the present invention wherein the seat 112 isadjustable upwardly and downwardly, the chair position may be optionallyadjusted between two image acquisitions so as to adjust the positions ofthe slices being imaged. In some embodiments, the movement may be veryslight, so as to compensate for effects of the collimators, which arediscussed in more detail hereinbelow. The chair position may also beadjusted upwardly or downwardly during an image acquisition.

As known to those of skill in the art, patient movement during imagingis a significant problem for most imaging systems. Most systems requirethe patient to lie on a narrow horizontal surface, in a ratheruncomfortable supine or prone position. This position is oftenuncomfortable for patients with back problems or for the many cardiacpatients that have difficulty breathing when lying flat. Often, thisresults in patient movement during the scan. In order to accommodate themoving detectors of current art systems, the patient must hold theirarms over their head for the duration of the imaging procedure. This isquite uncomfortable for many patients, particularly those with arthriticshoulders. Many patients experience fear or claustrophobia when lyingunder the large, metal detectors of current devices. Patients who areuncomfortable or fearful typically adjust their position in an attemptto become more comfortable. Such movement, when it occurs during animage acquisition, causes image artifacts, which may cause incorrectfindings and subsequent treatment. The problem is exacerbated by longscan times. The vertical positioning of a patient enabled by the presentinvention, as illustrated in FIGS. 1A and 1B, significantly improvespatient comfort and stability. It is much more comfortable for back andcardiac patients. The arms do not need to be held over the head. Theopen design of the present invention eliminates claustrophobia.Consequently, patient comfort and security is increased and movement isreduced. Also, some embodiments of the present invention allowsignificantly reduced scan times, thereby reducing the effects ofpatient motion.

III. General Discussion of 1-Dimensional Solid State Detector Modules(Strips)

FIG. 2 shows one embodiment of an individual detector module 150.Multiple (typically 64) individual modules are arranged in an arcsurrounding the patient. The arc may extend over a range ofapproximately 180 to 360 degrees. For cardiac SPECT, a preferredembodiment is approximately 180 degrees. The embodiment shown is asolid-state detector module sized for cardiac imaging. Other detectormodule embodiments are discussed below. As shown, the detector module150 is an elongated strip. Rectangular regions on the face of detectorindicate an array of individual solid-state detector elements 152, eachcomprising one pixel for data acquisition. In this embodiment, the arrayof detector elements is one-dimensional, i.e. 1×N, althoughtwo-dimensional arrays may also be employed. Multiconductor ribbon cable154 carries electrical signals from the detector elements to theelectronics that process the signals. Alternatively, some of theprocessing circuitry may be integral with or packaged by the detectorelements.

Each detector element 152 is operable to detect if a photon strikes it.Therefore, the overall detector 150 is operable to detect if a photonstrikes and is also operable to determine where along its length thephoton struck. Each detector element includes some semiconductormaterial, such as cadmium-zinc-telluride, with an electrode applied toopposing surfaces. An electrical potential is applied across theelectrodes. As will be clear to those of skill in the art, when a photonpasses through the front electrode and interacts with the semiconductormaterial, a small current travels between the electrodes. This currentis measured to sense the impact of photons.

While the present invention is initially described as using theabove-described detector elements, other embodiments of the presentinvention make use of other detector designs, as will be described inmore detail herein below.

IV. Aperture Arc—General Discussion

FIG. 3 shows the aperture arc 170 for an embodiment of the presentinvention optimized for cardiac SPECT. A single radiation detectormodule 172 is shown behind the arc to demonstrate relative positioning.As shown, the detector module is generally parallel to the longitudinalaxis. The arc 170 serves as a photon-blocking member and may be made oflead or a similar high attenuation material. The arc 170 is ofsufficient height to cover the radiation detection modules 172 situatedbehind it. The arc is of sufficient thickness (typically approximately 3mm) so as to effect essentially complete absorption of photons emittedby the patient. The arc is penetrated by a series of vertical apertureslots 174 which permit photons 176 aligned with the aperture slot topass from the patient through the slot to reach the detector modules.The slots are preferably generally parallel to the longitudinal axis ofthe patient.

In FIG. 3, the arc 170 is shown as a continuous member with generallyrectangularly shaped slots cut therethrough. In some embodiments, theslots are cut straight through, and have sides that are parallel to oneanother. Alternatively, the slots may be cut with angled sides such asshown in FIGS. 4-6. Each of these Figures illustrates a cross-section ofthe slot taken generally perpendicular to the slot. FIG. 4 illustratesan embodiment wherein the are 170 has tapered ends 171. The arc 170 maybe said to have a pair of opposing surfaces. The tapered points 171taper from each of these opposing surfaces to a point at approximatelythe center plane of the arc. For simplicity, FIGS. 4-6 illustrate aportion of the arc as being generally linear. However, as previouslydiscussed, it is actually arcuate.

Preferably, the arc 170 blocks substantially all of the photons exceptthose that pass through the slot 174. A certain thickness of photonblocking material, such as lead, is required to adequately block thesephotons. The tapered points 171 are thinner than the remainder of thearc. Therefore, it is preferred that they are formed out of a materialthat has even higher photon blocking ability, such as tungsten or gold,but could be lead. These tapered points 171 are joined to the materialthat typically forms the remainder of the arc 170. Alternatively, thearc, including the edges, could be all one material, such as lead. FIGS.5 and 6 illustrate alternative embodiments of tapered points 173 and175. In these embodiment, the edges of the slots taper either from thefront to the back or from the back to the front. As with the embodimentof FIG. 4, the points are preferably formed out of a material with ahigher photon blocking ability than the remainder of the arc. Thepointed edges of the slot are preferred, as they provide a moreconsistent apparent edge of the slot, independent of the angle fromwhich it is viewed. That is, a slot with squared-off edges may appearsubstantially narrower when viewed from an angle. By tapering the edgesof the slot, the slot has a more consistent effective width when viewedat a shallow or deeper angle. This is especially important in the designof the present invention since radiation may enter the aperture at asignificant angle. Alternatively, the “points” may be rounded.

In some embodiments of the present invention, it is preferred to haveslots with adjustable widths. This allows adjustment in the sensitivityand resolution of the imaging system. It may also assist in calibration.FIGS. 7 and 8 illustrate one approach to providing slots with adjustablewidths. FIG. 7 illustrates a cross-section of a portion of an arc 177with adjustable slot defining pieces 179 attached thereto. FIG. 8 showsa perspective view of one portion of an arc 177 with one adjustablepiece 179. By adjusting the positions of the pieces 179 relative to theremainder of the arc 177, the relative position and width of the slot178 may be adjusted. As with the embodiments of FIGS. 4-6, the thinnerportions of the end pieces 179 are preferably formed from a materialwith a higher photon blocking capability than the remainder of the arc177. The end pieces 179 are illustrated as having a front-to-back taper,but may have any of the shapes illustrated in FIGS. 4-6, or may providea more squared-off or rounded-off edge to the slot. Also, the end pieces179 are not required to be symmetrical. Additionally, a singleadjustable piece may be provided for each slot, with the other side ofthe slot being defined by a non-moveable edge. As will be clear to thoseof skill in the art, the interconnection between the end pieces 179 andthe arc 177 may be provided in a variety of ways, other than theapproach illustrates. Adjustment of the slot width may also be achievedin other ways, as will be clear to those of skill in the art.

V. Field of View

FIG. 9A diagrams (from above) the relative positions of the patientfield-of-view area 180, the aperture arc 182 and the detector modules184. It may be seen that the set of detector modules and the aperturearc are situated concentrically around the patient. One embodiment forcardiac imaging includes approximately 64 radiation detector modules184, each consisting of an array of individual elements or pixels. Inthis embodiment, the aperture arc 182 is positioned at a radius, a, ofapproximately 30 cm and the detector modules 184 are positioned at aradius, b, of approximately 40 cm. A patient field-of-view area with adiameter, c, of approximately 50 cm fits easily within the arc 182. Theaperture arc 182 and/or the set of detector modules 184 may be arrangedin a true geometric arc with common arc centers at the longitudinalaxis. Alternatively, either or both may be more ovalized or be arcuatewith non-shared arc centers. For example, the arc centers may bepositioned away from the longitudinal axis so as to increase the arcradii. It is also possible for the arc 182 and/or the set of modules 184to be non-arcuate. For example, either could be arranged as a series ofshort straight segments, or be partially arcuate and partiallynon-arcuate. Another example would be if either had different arc radiiat different radial positions so that the radius of curvature changesalong the “arc”.

Displacement means is provided for moving the aperture arc 182 relativeto the detectors 184. As will be clear to those of skill in the art,many different approaches may be used to move the aperture arc. Forexample, the aperture arc 182 may be connected by a worm gear or otherarrangement to a motor such that it can be rotated through a limitedangle about the longitudinal patient axis. As will be clear to those ofskill in the art, the arc may remain stationary with only the detectorsmoving. However, this approach is generally more complicated and costly.For purposes of processing the information from the scan, means are alsoprovided for accurately determining the position of the arc. As will beclear to those of skill in the art, many approaches to providing thismeans are available, including optical encoders and mechanical sensors.The sensing means may also be used for feedback control of thedisplacement means. A more detailed discussion of one approach to movingan aperture arc will be provided hereinbelow.

VI. Discussion of Sweep Due to Aperture Arc Movement

FIGS. 10A-C show overhead views of a single detector 190 and a smallsection 192 of the aperture arc. The Figures illustrate the relativeposition of the arc 192 and the detector 190 at three differentrotational positions of the aperture arc 192. At each position, theposition of the aperture slot 194 restricts the line of response of thedetector to a particular path 196, as shown. It can be seen that, as theaperture slot 194 moves in front of the detector 190, the line of sightof the detector fans across the patient, generating a multiplicity oflines of response or projections.

Since, as diagrammed in FIG. 9A, there are a multiplicity of detectormodules 184 and, as shown in FIG. 3, a multiplicity of aperture slots174, a multiplicity of detector lines of response are formed at eachrotational position of the aperture arc. FIG. 9B illustrates a smallsubset of the lines of response 200 obtained from a few of the detectors202 as the aperture arc 204 is rotated. The aperture slots themselvesare not shown in this Figure, for simplicity. A diagrammatic “slice” 206through the patient's chest is shown, indicating that a full set ofprojections of the heart, sufficient for tomographic reconstruction, isobtained in this manner.

The aperture arc preferably moves continuously, such that the lines ofresponse “sweep” over the field of view. Alternatively, the aperture arcmay move in discrete steps, with imaging occurring with the arc stoppedat each of the steps.

VII. Each Detector Illuminated by Only a Single Aperture Slot

All detectors preferably “look through” only one slot at all times. Slotspacing is determined such that each detector is illuminated by only oneslot at a time. Overall photon detection efficiency is proportional tothe number of slots in the aperture arcs. The maximum number of slotspermissible, n_(slots), is a function of the angle φ_(arc), representingthe maximum angel of incidence of a usable ray at an aperture slot, theradius of the detector arc and the minimum length of arc on the aperturearc such that a

$n_{slots} = {\frac{\pi \cdot \frac{\phi_{arc}}{2\pi}}{\theta_{A}/2} = \frac{\pi \cdot \frac{\phi_{arc}}{2\pi}}{{\sin^{- 1}\left( \frac{R_{O}}{R_{A}} \right)} - {\sin^{- 1}\left( \frac{R_{O}}{R_{D}} \right)}}}$given length of arc θ_(A) on the aperture arc such that a given detectorwill only see the patient field-of-view through one slot at a time(θ_(A)):where R_(O) is the radius of the patient, R_(A) is the radius of theaperture arc and R_(D) is the radius of the detector arc. The aperturearc need only be rotated by the interval between slots,φ_(arc)/n_(slots), to provide a full set of angular projections.

For one embodiment of the present invention, the radius of the patientR_(O), is assumed to be a maximum of 22 cm, the radius of the aperturearc R_(A), is 30 cm and the radius of the detector arc, R_(D), is 45 cm.The detector arc and aperture arc span an angle, φ_(arc), of 180 degreesand the minimum length of the arc, θ_(A), is 36 degrees. For thesevalues, the equation provides that five slots are the maximum number ofslots to avoid any detector looking through more than one slot at atime. Consequently, the aperture arc need only rotate through an angleof 36 degrees to provide a full set of angular projections.

The above equation and solution assumes that the slots are equallyspaced along the arc, and separated by an angle of 36 degrees. As willbe clear to those of skill in the art, the critical issue is actuallythe angular separation between the slots, which determines the number ofslots. Referring again to FIG. 3, the arc is shown with five slots, onewhich is hidden in the bend, due to the angle of view in the Figure.

While the above equation and discussion leads to the conclusion that 5slots are needed, with a separation of 36 degrees between the slots, theaddition of a 6^(th) slot is beneficial. FIG. 11A diagrammaticallyillustrates the present invention with a plurality of detectors 195disposed in an arc, an aperture arc 196 with five apertures 197, and afield of view 198. The arc 196 is shown at the extreme clockwiseposition. Assuming photons of interest may originate from anywhere inthe field of view, projection rays are drawn to show how the field ofview is “projected” onto the arc of detectors 196. As shown, somephotons are projected to a position clockwise of the last detector, andtherefore do not contribute to the image. Likewise, a number of thedetectors at the counterclockwise end are “out of view” of the apertureat the counterclockwise end of the aperture arc 196, and are thereforeunexposed with the arc in this position. Unexposed detectors represent aless-than-optimal system efficiency.

FIG. 11B illustrates the aperture arc at the midpoint of its travel. Asshown, at this position, the projections through all apertures 197coincide with the positions of the detectors 195, so that no photons arewasted and no detectors are unexposed.

FIG. 11C illustrates the aperture arc 196 at the extremecounterclockwise position. In this position, detectors at the clockwiseend of the detector assembly are unexposed, and some photons passingthrough the apertures at the counterclockwise end go undetected.

One solution to this problem is to provide a larger number of detectors.However, this increases the size of the imaging section, anddramatically increases the cost of the device. A preferred solution isillustrated in FIG. 11D. The aperture arc 212 now has 6 slots 214projecting photons onto the detectors 216, from the field of view 218.The spacing between these slots is unchanged, however, from thatdetermined by the equation above (36 degrees in this example). FIG. 11Dillustrates the arc 212 at the extreme clockwise position. As shown, alldetectors are illuminated due to the addition of the sixth slot. FIG.11E illustrates the arc 212 at the midrange of travel, and FIG. 11Fillustrates the arc at the extreme counterclockwise position. Again, alldetectors 216 are illuminated at all positions, thereby increasingphoton collection efficiency. The addition of the “extra” slot, resultsin a perfect match of incoming photons to the length of the arc ofdetectors. In this arrangement, all detectors are illuminated via theaperture slots at all times, thereby optimizing photon detectionefficiency.

VIII. Diagonal Apertures

Referring again to FIG. 3, the slots 174 are shown as generally verticalslots. That is, they are parallel to the longitudinal axis of the fieldof the view. According to further aspects of the present invention, theslots may be diagonal as shown in FIG. 12. FIG. 12 illustrates anassembly including an aperture arc 207 with diagonal apertures 208defined therethrough. The diagonal apertures are illustrated as beingdefined by adjustable side pieces 209, but may alternatively be providedby slots cut into the arc 207. Also, as with the earlier embodiments ofslots, the slot edges may be tapered in a variety of ways, including anyof the previously disclosed shapes. As will be clear to those of skillin the art, multiple apertures are preferred, arranged in intervalsalong the arc 207. Only two apertures 208 are illustrated in FIG. 12,for simplicity. However, additional apertures are preferred. FIG. 12illustrates additional aspects of the present invention, which will bediscussed hereinbelow with respect to collimator design. The angledslots or apertures 208 may be provided at a variety of angles rangingfrom slightly angled from “vertical,” to nearly horizontal. As a furtheralternative, the slots may be completely “horizontal” with respect tothe patient axis. The apertures may also be angled in the oppositedirection to the angle illustrated in FIG. 12.

In embodiments of the present invention where the apertures are“vertical” and the collimators are horizontal, or vice versa, theresolution is different in the vertical and horizontal directions.According to one preferred embodiment of the present inventions, theapertures are angled at approximately 45 degrees one direction, and thecollimators are angled at approximately 45 degrees the other direction.By angling the apertures and the collimators relative to the transaxialimaging plane, the overall resolution experienced at the imaging planeis made essentially isotropic, i.e. similar in all directions. This isdesirable in some applications, particularly if the reconstructed dateis to be reformatted along obliquely angled planes.

IX. Collimators

Referring again to FIGS. 3 and 11A-F, the aperture are and the set ofdetectors provide projection data collimated within the transaxialplane, but not collimated longitudinally. For this reason, the inventionpreferably provides a set of longitudinal or cross-plane collimators, asshown in FIG. 13. As will be clear to those of skill in the art, thecollimator design illustrated in FIG. 13 is designed for use with the“vertical” aperture arc, such as shown in FIG. 3. The longitudinalcollimators consist of a stack-like series of arc-shaped vanes 220arranged as shown and located concentrically to the arc arrangement ofdetectors 222 as shown. The aperture arc is omitted from this figure,but is located concentrically to the longitudinal collimator vanes. Thevanes are preferably mutually parallel and generally perpendicular tothe longitudinal axis of the patient. The vanes are sheets or panels oflead or similar attenuating material and may be separated by spacers ofradiolucent plastic foam or similar material (not shown). The number,size, and thickness of the vanes may be varied depending on theapplication.

FIG. 14 is similar to FIG. 13 but with the addition of the aperture arc230. It may be seen that each individual detector element pixel) of eachdetector 232 has a unique line-of-response 234 directed into the patientfield-of-view by the combined collimating effects of the aperture arcslots 236 and the longitudinal collimating vanes 238.

As will be appreciated by those of skill in the art, it is preferredthat the vanes 220 be provided in a plane that is generallyperpendicular to the apertures in the aperture arc. In the embodiment ofFIGS. 13 and 14, collimators vanes may be considered to be “horizontal,”since they are perpendicular to the “vertical” patient axis. Referringagain to FIG. 12, it can be seen that the collimators 210 are angled soas to be generally perpendicular to the angled aperture. Only fivecollimating vanes 210 are illustrated in FIG. 12, in order to avoidcluttering the drawing. However, it will be appreciated that the vanesare provided along the entire assembly, as indicated by the arrows. Ifthe apertures are angled at other angles, the vanes 210 may also beangled so as to remain perpendicular thereto. Alternatively, thecollimator vanes 210 and apertures 208 may be at angles to one anotherother than perpendicular.

X. Resolution and Efficiency

The in-plane resolution of a system according to the present inventionis determined by the radii of the detector and aperture arcs, R_(D) andR_(A), the distance, Dist, of the object from the aperture arc, and thewidths of the slots and the detector elements, W_(slot) and W_(det)respectively:

${resolution} \approx {W_{slot} + \frac{{Dist} \times \left( {W_{slot} + W_{\det}} \right)}{\left( {R_{D} + R_{A}} \right)}}$

FIG. 15 plots the resolution at different depths (distance from thecollimator to the point of interest in the patient) of the presentinvention versus a traditional parallel-hole collimator. The slotted arcsystem is assumed to have a slot width of 2.4 mm, a detector width of 4mm and other parameters as discussed with respect to FIG. 4. Theparallel-hole collimator for which data is plotted has a hole diameterof 2.2 mm and a collimator thickness of 3 cm.

The detection efficiency of the slotted aperture system is proportionalto the detector solid angle, Ω, for a point source at the center of thefield-of-view and may be calculated based on Rogers (EEE TIMI, vol.MI-1, pp. 63-68, 1982) as: where r_(obj) and r_(D) are thefull-width-half-maximum object and detector resolution respectively,p_(det) is the detector packing fraction and f is the fraction offrontal area closed by the longitudinal collimating vanes. In theconfiguration of this invention, f=vane thickness/vane separation.

As the aperture arc moves to differing positions relative to thedetectors, the apparent width of the aperture slots will vary as afunction of the sine of the angle between the slot and the detector.Since the apparent width of the detector as viewed from the slot alsochanges according to a similar function, the overall detectionefficiency will vary as a function of the square of the sine of thedetector-slot angle. The exact function will depend on the photoncross-section of the detector element (a function of detector thickness)and on the photon cross-section of the slot aperture. This variation ofdetector sensitivity with slot position is easily mapped for a givendetector and may be corrected for in software in a manner similar to thedetector uniformity corrections routinely performed in traditional gammacameras.

It is to be noted that imaging systems constructed according to themethods of this disclosure are relatively insensitive to the structuredimage artifacts seen in rotating gamma camera SPECT systems whennon-uniformities of detector sensitivity exist. In the systems describedhere, the reduced count sensitivity caused by a particular, relativelyinsensitive, detector element is spread across the entire image plane,rather than appearing as the structured “ring” or “arc” artifacts seenin traditional systems. Such artifacts frequently trouble presentartifact systems.

$\Omega = {n_{slots}{\frac{1}{R_{D}^{2}}\left\lbrack {{\sqrt{r_{obj}^{2} - r_{D}^{2}} \cdot \frac{1}{R_{A}}}\sqrt{\left\lbrack {r_{obj}\left( {R_{D} - R_{A}} \right)} \right\rbrack^{2} - \left\lbrack {R_{A}r_{D}} \right\rbrack^{2}}} \right\rbrack}{fp}_{\det}}$XI. Collimator Construction

As will be appreciated by those of skill in the art, the construction oflead collimators presents significant challenges. Lead has a very highdensity, but is not particularly stiff or strong. Therefore, vanes oflead are heavy and vulnerable to damage. In traditional parallel holecollimators, the vanes are made very thin and define a plurality ofsmall parallel holes. The depth of the holes in the collimators issomewhat limited by the strength and stiffness of the lead material.That is, if a collimator is to be constructed that has more than aparticular depth, the thin lead vanes may actually sag over time,destroying the usefulness of the collimator. Similar considerationsapply to the present invention. The collimating vanes, such as 220 inFIG. 13 and 210 in FIG. 12 are large and heavy, thereby presentingchallenges to how to adequately support the individual vanes.Additionally, it is important that the individual vanes be accuratelypositioned and aligned.

A further inventive aspect of the present invention is a designproviding a collimator with parallel lead vanes that are supported bybeing formed in a stack with sheets of radiolucent material disposedbetween each lead vane. FIG. 16 illustrates a portion of a parallel vanecollimator constructed according to this aspect of the presentinvention. FIG. 16 also illustrates a portion of a support assembly,including a lower support member 240 and an upper support member 242.

FIG. 17 shows the lower support member 240 and upper support member 242in their entirety, according to one embodiment of the present invention.However, FIG. 17 does not illustrate the collimation assembly inside ofthe support assembly. Referring to FIG. 17, the lower support member 240and upper support member 242 form part of a support assembly 244. Thissupport assembly 244 forms part of the imaging arc 106, as shown inFIGS. 1A and 1B. It wraps about the patient field of view, illustratedat 245 in FIG. 17. When assembled, the imaging arc includes the supportassembly 244, the parallel vane collimating assembly supported therein,single or multiple detectors, and the aperture arc. It is alsopreferably clad in a housing so as to protect the internal workings, andprovide an aesthetically pleasing exterior appearance. One end of thesupport assembly 244 is interconnected with the chair base 108 forsupporting the imaging arc. This may be accomplished in a variety ofways. Alternatively, an additional support may be provided mid-arc.

Referring again to FIG. 16, a portion of the parallel vane collimatingassembly is shown at 246. The collimating assembly includes sheets orpanels of lead 248 with sheets or panels of radiolucent material 250separating the lead sheets 248. The collimator assembly may be formed bystacking a lead sheet, and then a radiolucent sheet, and then repeatingthe process until a sufficiently tall stack is formed, as shown. Theradiolucent material maintains the relative positioning of the leadsheets, and prevents any sagging or movement of the lead sheets.Preferably, a compression panel or upper support plate 252 is providedon top of the stack of lead sheets and radiolucent material, and belowthe upper support member 242. Biasing devices, such as threaded member254 are then provided to press downwardly on the compression panel 252.This compresses and stabilizes the stack 246. Preferably, a thicker leadsheet, or other photon blocking material 253 is provided at the top andbottom of the stack, to block photons from entering the top or bottom ofthe collimator assembly.

As will be clear to those of skill in the art, a modified version ofthis assembly procedure may be used to construct a collimator assemblysuch as shown in FIG. 12. According to a further aspect of the presentinvention, a related approach may be used to form parallel holecollimators. That is, a parallel hole collimator may be formed usingradiolucent material filling the holes in the parallel hole collimator,to thereby support the collimator vanes. Parallel hole collimators areoften damaged in use, because of the fragility of the lead septaebetween the holes. According to the present invention, the holes of thecollimator may be filled with a radiolucent material as it isconstructed. This turns the parallel hole collimator into substantiallya solid block, which is more resistant to damage. Also, this allowsdeeper and/or thinner vanes to be formed and supported than wouldotherwise be practical.

Referring again to FIG. 17, an alternative approach to forming aparallel vane collimator assembly according to the present invention maybe provided by allowing the upper and lower support members 240 and 242to be tensioned against each other, such as by tensioning members 256.That is, the alternating stack of lead panels and radiolucent panels maybe placed on the lower support member 240, covered by upper supportmember 242, and compressed using compression or tension members 256.Those of skill in the art will appreciate that the parallel vanecollimator according to the present invention is very heavy, andtherefore the cantilevered arc support assembly bears a substantialload. FIG. 18 illustrates that the support assembly may include aplurality of angled tension members 258, either angled to the left asshown, or angled to the right, or both. The tension members act likebicycle spokes in providing structure and support. They also allow asubstantially open back to the arc for access to the electronics and forcooling.

FIG. 19 provides a cross-sectional view of a portion of the imagingsection of the present invention. It illustrates the lower supportmember 240, the upper support member 242 and the lead sheets 248positioned therebetween. The radiolucent material is not illustrated inthis view. However, an electronics package or detector array fordetecting incoming photons is illustrated generally at 260. Thisdetector array will be discussed in more detail hereinbelow.

The design of the present invention provides advantages heretoforeunavailable with respect to collimator design. Traditionally, collimatordesigners have limited the depth to width ratio of the collimator holes.That is, the holes defined by the collimator may be considered to have afront-to-back depth and a side-to-side or top-to-bottom width. (In aparallel hole collimator, a side-to-side and top-to-bottom widths aretypically the same. In the present invention, the “side-to-side width”is a function of the size of the aperture in the aperture arc, while thetop-to-bottom width is a function of the spacing between the parallelvanes.) In the prior art, a depth-to-width ratio of less than 10:1 hasbeen considered optimal. In fact, the literature has stated that a 10:1ratio is almost equivalent to an infinitely large ratio. In other words,excepted theory has taught against depth-to-width ratios over 10:1.Additionally, prior art designs for collimators have made itextraordinarily difficult to create a depth-to-width ratio that is verylarge. Deep collimators suffer from structural integrity issues. To geta high depth-to-width ratio in prior art designs requires vanes that aretoo thin and tall to be self supporting. So, practicality also taughtaway from high depth-to-width ratios.

The present invention departs dramatically from the prior art approach.In one embodiment of the present invention, the lead sheets have athickness of approximately 2 mm, as indicated at A in FIG. 16. Theradiolucent sheets have a thickness of approximately 4.5 mm. Therefore,the “gap” between adjacent lead sheets is approximately 4.5 mm. In thissame embodiment, the front-to-back depth of the lead vanes 248, as shownat C in FIG. 19, is approximately 150 mm. In this embodiment, thedepth-to-width ratio is greater than 33:1. In a more preferredembodiment of the present invention, the lead vanes have a thickness ofapproximately 1.25 mm. However, the gap remains the same atapproximately 4.5 mm. Therefore, the depth-to-width ratio remains thesame. According to the present invention, depth-to-width ratios greaterthan the prior art maximum of 10:1 are preferred. Depth-to-width ratiosgreater than 20:1 are more preferred. Depth-to-width ratios over 30:1are even more preferred.

According to the present invention, it is also preferred that thethickness of the lead vanes be greater than 0.5 mm. A thickness ofgreater than 0.75 mm is more preferred, a thickness of 1 mm or more iseven more preferred, and a thickness of at least 1.25 mm is mostpreferred. These thicknesses also depart dramatically from the priorart. Prior art high resolution parallel hole collimators typically havelead vanes with a thickness of 0.2 mm or less, and significant efforthas been expended to obtain thinner and thinner lead vanes.

The use of substantially greater depth-to-width ratios than used in theprior art, as well as the use of substantially thicker lead vanes,provides significant advantages that have not been recognized orappreciated in the prior art.

In SPECT imaging, it is important to accurately determine the directionfrom which a photon is traveling, the energy level of the photon, andthe number of photons coming from that direction. These photons havesufficient energy to penetrate lead if it is not sufficiently thick. Inprior art parallel hole collimators, the thin lead vanes are typicallytoo thin to stop many of the photons from passing therethrough.Therefore, a photon that strikes a particular area cannot be assumed tohave traveled straight down the hole adjacent that area. Instead, thephoton may have originated in a different hole and penetrated the leadvane in between the adjacent hole and the hole in which it is sensed.Consequently, accuracy is sacrificed. This contributes to blur in theresulting image. The depth-to-width ratio of the holes in the collimatoralso has an effect on the resolution of the imaging device. If acollimator hole is short and wide, a photon may enter that hole at anangle significantly off from the axis of the hole. If the hole is deeperand narrower, the range of angles of incoming photons that travel justdown that hole is much narrower.

In the present invention, the use of substantially thicker vanes and theuse of a collimator with a very high depth-to-width ratio, both lead tosubstantially increased accuracy or resolution. Because the vanes arethick and the depth is very high, any photon that reaches the sensor atthe back of the collimator can be assumed to have passed through theaperture in the aperture arc and between the adjacent lead vanes. Inother words, each photon “count” is a good count.

The prior art also tends towards the use of much smaller gaps than inthe present invention. Experimentation with the present invention hasshown that larger gaps, on the order of 4 or 4.5 mm, along with thickerlead vanes lead to higher efficiency and resolution. As a further aspectof the present invention, the use of gaps greater than 2 mm ispreferred, with gaps greater than 3 mm being more preferred, and gaps of4 or more mm being most preferred.

Referring again to FIG. 19, the sensor array 260 is positioned adjacentthe back of the collimating assembly. In some embodiments, theindividual sensors are positioned immediately adjacent the rearmost endof the vanes, while in other embodiments the sensors are spaced from theback of the vanes by a short distance. Increasing the gap between theback of the vanes 248 and the sensors reduces some of the effective darkarea caused by the photons that are blocked by the vanes. In onepreferred embodiment, the sensors are spaced from the back of the vanesby 2 to 3 mm.

XII. Extension Flaps

As shown in FIGS. 1 and 4, for an embodiment optimized for cardiacimaging, the use of an arc shaped imaging apparatus allows the patientto easily enter and leave the imaging system. As the aperture arcrotates however, it will extend slightly into the open area of the arc.The invention therefore optionally provides for pivoted Extension Flapsto be located at one or both ends of the aperture arc, as shown in FIGS.20A and 20B. This figure shows one end of the aperture are 300 thatincludes an extension vane 302 extending its length. FIG. 20A shows theaperture arc 300 and vane 302 at one extreme of the arc's movement andFIG. 20B shows them at the other extreme. Extension vane 302 is movablyattached to the aperture arc by hinge 304. Pivot rod 306 is located inthe path of the vane such that, as the extension vane is pushed againstit by the movement of the aperture arc, the extension vane is caused topivot away from the patient as shown in FIG. 20B. This minimizes theextension of the arc or vane into the opening while maintainingshielding of the detectors from unwanted external radiation.

Referring now to FIG. 21, one preferred construction of the aperture arcis illustrated. The aperture arc is shown at 310, being supported on thesupport member 240, which forms the bottom part of the support structureof the imaging arc. In this embodiment, the aperture are 310 is formedfrom individual arcuate panels 312 that are positioned adjacent oneanother so as to provide an aperture 314 therebetween. The width of theaperture 314 may be determined by the relative positioning of the panels312. The aperture arc 310 is supported in a track in the support member240 and moved by a drive motor 316, which drives a series of belts andpulleys.

XIII. Detector Variations

Turning now to detector or sensor designs, a variety of approaches maybe used with the present invention. FIGS. 2 and 3 illustrate stripdetectors that may be considered one-dimensional linear arrays.Two-dimensional arrays are also provided in this invention. Such arraysmay be provided as integral units or may be approximated by placing twoor more one-dimensional arrays in close proximity. The overallsensitivity of the imaging system is linearly proportional to thedetector surface area available.

Referring to FIGS. 22-24, three views of a preferred embodiment of asensor assembly for use with the present invention is generally shown at320. As best shown in FIG. 23, the assembly 320 includes threetwo-dimensional sensor arrays 322, 324, and 326. Each sensor array, inturn, is formed of a series of sensor modules, such as 328 in FIG. 24.The sensor modules are solid state CZT (Cadmium Zinc Telluride), oralternatively, Cadmium Telluride may also be used. FIG. 25 illustrates across-sectional view of one of the sensor modules 328. The module has acentral body of CZT 330 with multiple small, thin, square electrodes 332on the front face. A larger electrode is provided on the back surface,and a chip for processing data signals from the sensor is provided onthe back at 336. Photons strike the front surface of the sensor module328 and are sensed by the module. FIG. 26 illustrates an alternativeembodiment wherein a chip 338 is only half covered by sensing materials340. FIG. 26 also illustrates the configuration of the electrodes 342 onthe face of the module.

FIGS. 22 and 24 illustrate cooling manifolds 346 for the sensingassemblies.

As known to those of skill in the art, solid state photon sensors aredifficult to produce without internal flaws. Referring to FIG. 25, thebody of CZT material 330 is a crystal that may develop flaws duringcreation or manufacturing. If the body 330 does not have flaws, a photonpassing through the front face and into the CZT body 330 enables thepresence of this photon to be sensed by the electrodes 332 and 334. Asshown in FIG. 26, the electrodes 342 define a two-dimensional grid.Consequently, the location of the photon strike may be determined bydetermining which electrode senses the presence of the photon. If theCZT is flawed, it may have dead spots, where a photon strike is notsensed. Typically, electrodes on the front of the CZT body are sized andspaced so that one electrode is responsible for sensing one “pixel” ofinformation. Typically, a pixel size is chosen and equal to the desiredresolution of the sensing system. In cardiac sensing, it is preferred tohave resolution of approximately 4 to 4.5 mm. Therefore, the electrodeswould typically be arranged on 4-5 mm centers such that one electrode isresponsible for each “pixel.” If the CZT has a flaw, the flaw may causea dead pixel, which can seriously affect image quality.

According to a further aspect of the present invention, the desiredresolution, in this case, 4 to 4.5 mm, is subdivided into smallersegments and smaller electrodes are used. In FIG. 26, box 350 representsan area that is approximately 4 to 5 mm wide and tall. However, ratherthan having a single electrode in this area, this “macro pixel” issubdivided into four pixels, each with its own electrode 352. If the CZTunderlying the macro pixel 350 has a flaw, the flaw will typically leadto only a single bad pixel associated with one of the electrodes 352.For example, one of the four electrodes may be associated with a portionof the CZT that has no sensitivity, reduced sensitivity, or, in rarecases, increased sensitivity. The sensor module can then be calibrated,and the data from the four electrodes 352 processed so as to providemeaningful data from the macro pixel 350. For example, if one electrodeis associated with a pixel that is dead, the output from the remainingthree pixels may be combined, and multiplied by ¾ to obtain an outputfor the macro pixel 350. In this way, a sensor module with a CZT bodywith some flaws is still useable. In the module of FIG. 26, theelectrodes 352 preferably have a side-to-side and top-to-bottomdimension of approximately 2.46 mm, and spacing between adjacentelectrodes of approximately 0.04 mm. In another preferred embodiment,especially optimized for cardiac use, the electrode-to-electrode pitchis approximately 2.25 mm. Referring again to FIG. 19, the sensorassembly 260 is shown adjacent the rear of the lead vanes 248. FIG. 27illustrates a view of the sensor arrays 360 as viewed through the vanes362. In some embodiments, the pitch between the vanes 362 is not evenlydivisible by the pitch between the electrodes 364. For example, in oneembodiment, the pitch between the vanes 362 is approximately 6.5 mm,while the pitch between the electrodes 364 is approximately 2.5 mm. Inorder to avoid more patterns due to the alignment between the vanes andthe pixels, it is desirable that the number of pixels in each gapbetween the vanes is approximately the same. Because the vane pitch isnot a multiple of the pixel or electrode pitch in this embodiment, thesensor arrays 360 are arranged such that they are centered on the middlevane 366. As shown in FIG. 27, this arrangement prevents an electrode,and hence a pixel, from lying directly behind one of the vanes 362.

This invention also provides for radiation detectors constructed fromscintillation materials such as sodium iodide or cesium iodide withassociated photomultiplier tubes or other photo-detectors such as solidstate photodiodes. FIG. 28 shows one embodiment of a scintillation-baseddetector module 400. This embodiment includes a cylindrical crystal 402of scintillation material clad in a radiolucent, light-reflectivecovering 404 such as aluminum. The covering 404 is open at both ends ofthe cylinder. Affixed to each end, via optical coupling material, is alight detector such as a photomultiplier tube, photodiode, or otherphoto-detector (not shown). The position of scintillation eventsoccurring within the scintillation material is determined by the ratioof outputs of the two photo-detectors, thus providing longitudinalposition sensing within the detector. This embodiment is extremelyinexpensive to produce, but has the disadvantage of a variable photondetection efficiency across its horizontal dimension caused by thevarying scintillator thickness over its circular cross-section. Thiscauses a deviation of the detector's response function from a pure rectfunction, thus slightly degrading spatial resolution.

FIGS. 29A-C show more efficient embodiments of a scintillator-baseddetector, consisting of a rectangular bar 420 of scintillator materialclad in a radiolucent, light-reflective material 422 such as aluminum.In FIG. 29B, the cladding is open at the top and bottom so as to permitplacement of photo detectors 424. In the alternative embodiment shown inFIG. 29C, the cladding is open at the rear of the module so that two ormore photo-detectors 426 can be affixed. In either case, thephoto-detectors are considered to be adjacent the ends of thescintillation material so that they can locate the position of ascintillation event.

FIG. 30 shows a piece of scintillator material 430 with a trapezoidalcross section clad in reflecting material 432, similar to the previousFigures. As with the embodiments of FIGS. 29A-C, the photo-detectors maybe affixed on either the top and bottom of the module or at the rearface. The embodiment with the trapezoidal cross section has theadvantage of presenting a more uniform cross-section to incomingradiation, but is more costly to manufacture. That is, radiation comingat an angle to the front face still encounters the full depth of thescintillator material.

Axial resolution of the tomography system is directly dependent ondetector width, as described above. Specifically, narrower detectorsincrease the axial resolution of the system. As detector width narrows,however, photon detection efficiency drops because photons striking thefront face of the narrow detector may scatter out of the detectormaterial before they have deposited all of their energy. According tothe present invention, the efficiency of a high resolution elongatedstrip of scintillation material may be improved by masking a portion ofits front face. FIG. 31A shows a detector configuration 440 based on arectangular piece of scintillation material. FIG. 31B shows a detectorconfiguration 442 based on a cylindrical piece of scintillationmaterial. FIG. 31C shows a detector configuration 44 based on a piece ofscintillation material with a trapezoidal cross section. In each ofthese embodiments, in addition to the reflective cladding 446, thescintillator is clad in an additional masking layer 448 of lead,tungsten or similar high-attenuation material. This outer masking orshielding layer is configured to have a narrow vertical opening 450 ofthe dimensions desired for the detector cross-section. Once photons havepassed through the opening and struck the scintillator, furtherscattering is more likely to occur within the larger volume ofscintillator located behind the opening 450 in the mask 448 rather thanscattering outside the scintillator material. If desired, an additionallayer of low-Z material (not shown) may be interposed between thecladding and the shielding layers so as to absorb secondary lead x-raysemitted by the mask 448. As will be clear to those of skill in the art,the detectors shown in FIG. 31D have the improved efficiency of widerdetectors with the higher resolution of narrower detectors. Similarmasking can be applied to solid-state detectors, such as shown in FIG.2, resulting in similar advantages.

Referring to FIG. 34, a similar masking approach may be applied to atwo-dimensional piece of scintillation material to form a detector 452with the benefits described above. Specifically, a piece ofscintillation material 454 has mask of lead applied in strips 456 to itsface. Narrow vertical openings 458 are left to allow entrance of photonsaligned with the openings. Like with the embodiment of FIGS. 14A-14C,this gives increased accuracy. Photo-detectors 459 are positioned behindthe scintillation material 454 and are capable, by means such as “Angerlogic”, of detecting where a pulse of light occurs. Because a portion ofthe face is masked, the electronics “knows” that the photon did notstrike in the masked areas and can therefore more precisely pinpoint thelocation of the strike. The masking off of certain portions of thedetector surface reduces, in effect, the positional uncertainty of agiven pulse of light, thus permitting its position to be determined moreaccurately and precisely.

FIG. 32 shows details of construction of a bar-shaped, masked detectormodule 460 as described in the previous Figures but with thephoto-detectors 462 attached at the rear face through use of opticalcoupling material 464. A similar masking configuration may be used withsolid-state detector modules.

As will be clear to those of skill in the art, photo-detectors ofvarious types are somewhat costly. Therefore, it is desirable to reducethe number required. According to another embodiment of the presentinvention, a pair or more of optical fibers may be attached to each ofthe scintillation-based detectors, with one fiber connected to each endof the detector. The fiber may be connected to the top and bottom and/orto the back face adjacent the top and bottom. The optical fibers maythen be routed to a photomultiplier of the type have positionsensitivity. These readily available multichannel photomultipliers arecapable of providing distinct outputs for a multiplicity of locationsacross the face of an individual tube. Such a photomultiplier can thensense light pulses from a large number of optical fibers running fromvarious detectors. In this way, the total number of photo detectors isreduced. A similar approach may be applied to two-dimensionalscintillation based detectors. Rather than using photodetectors mountedto the rear of the material, multiple optical fibers may be used toroute the light to multichannel detectors.

As previously discussed, the pieces of scintillation material that formthe core of a scintillation based detector are clad in a radiolucent,light reflecting material such as aluminum. This increases thebrightness of the pulse of light as perceived by the light detectors.However, in some situations, this reflectivity may interfere with theability of the light detectors to determine the longitudinal positionwhere the photon struck the scintillation material. Therefore, it may bebeneficial to reduce the reflectance of one or more surfaces of thescintillation material. For this purpose, the surface may be roughenedprior to cladding, the cladding may be roughened in certain areas, or alower reflectance coating may be applied to either the scintillationmaterial or the cladding. Alternatively, it may be desirable to vary thereflectance along the length of the reflector. For example, a roughedstrip on one surface of the scintillation material may vary in widthalong the length of the detector. The strip could be narrow in thecenter, so that reflectance remains high, and wider near the ends sothat reflectance is reduced. This increases the likelihood of eventsnear the center being detected at the ends.

XIV. Detectors and Arc May Both Move

If the spacing of detector modules is sparse, gaps may be seen in thepattern of angular sampling provided by this system. The importance ofsuch gaps depends on the number of angular “bins” of data obtained asthe aperture arc moves. In addition, the significance of any artifactscaused by incomplete angular sampling depends on the clinical setting.If such artifacts are objectionable, this invention optionally providesfor a means (FIG. 33) of rotation of the arc of detector modules 500through a limited angular range 502, such motion occurring eithercontinuously or in a limited number of discrete steps. The range ofmotion of the detector arc is equal to the spacing between detectors. Ateach step of detector motion, the aperture arc 504 is moved through itsrange of motion 506. In this manner, a full set of angular projectionsmay be obtained with even sparse detector population.

As another alternative, a tomography system according to the presentinvention may be provided with a reduced number of detectors to reducethe cost of the system. This system would have either reduced resolutionor would require an increased scan time. Later, the system may beupgraded by adding additional detectors at positions between theexisting detectors.

XV. Calibration

As known to those of skill in the art, nuclear medical imaging devicesrequire regular calibration. With typical parallel hole gamma cameras, asheet of material with radioactive substance on one side is positionedagainst the face of the collimator in order to perform a calibration.The present invention creates different challenges. A tubular radiationsource could be positioned at the patient axis. However, calibrationwould then be very time consuming, since it would require long exposuretimes at each arc position over a number of positions. This would alsolead to unacceptable levels of radiation in the room during thecalibration process. FIG. 35 presents a preferred calibration approach.A portion of an aperture arc is shown at 510 with an aperture at 512. Acalibration member 514 is shown positioned adjacent the aperture 512. Itis arc-shaped, and may have a smaller radius and curvature than shown.The inside surface 516 has a radioactive material on it, and ispositioned such that the radioactive material causes photons to travelthrough the aperture 512. This results in radioactive material coveringthe entire field of view of the sensors that can “see” the aperture.Obviously, multiple calibration members 514 are used, with one beingplaced at each of the apertures. This allows a rapid calibration of thedevice, allows for compact storage of the calibration devices, andminimizes the exposure to radiation.

XVI. Alternative Configurations

The previously described embodiments of the present invention havespecified that the detector sensor arrays, the collimators, and theblocking member each be arcuate in shape. As will be clear to those ofskill in the art, other shapes are also possible. For example, thedetectors may be laid out in a rectangular or square arrangement. Theblocking member and the collimators could be shaped likewise. As anotherexample, sets of either strip or two-dimensional detectors may bearranged in straight rows at various positions around the field of view.This approach is shown in FIG. 36 using two-dimensional detectors 520.Each row of detectors 520 has a blocking member 522 in the form of astraight sheet positioned in front of it. The blocking member 522 hasapertures, such as slots 524, defined through it and moves as shown byarrows D so that lines or response are swept across the field of view.Collimators, as discussed with other embodiments herein, may also beprovided. As a further alternative, the detectors, either strip ortwo-dimensional, may be arranged as shown in FIG. 36 and an arc or ringshaped blocking member may be used. This arrangement, or the arrangementof FIG. 36 may cover an are between 180 and 360 degrees. In theseembodiments, if two-dimensional detectors are used, conventional largetwo-dimensional detectors, as used in gamma cameras may be cut intoseveral, preferably four, pieces to provide the smaller two-dimensionaldetectors necessary for these embodiments. This reduces the total costof components.

Depending on the application, the system of the present invention mayinclude other accessories. For example, in cardiac work, it may bedesirable to stress the heart by having the patient perform an exercise.For this purpose, the system may include a bicycle ergometer that iseither permanent or detachable. Also, the system may include anelectrocardiogram and/or a built in cardiac defibrillator. Also, anintravenous infusion pump may be included or be attachable.

XVII. Structural Considerations

FIG. 37 shows a support assembly for an imaging arc 610 similar to theone shown in FIG. 18, However, FIG. 37 shows the arc having diagonalspokes or tension members running in an additional directional betweenan upper support member 612 and a lower support member 614. The spokesmay be considered to include a vertical set and a first and seconddiagonal set. A horizontal spoke representative of spokes in thevertical set is labeled 616. A spoke representative of the spokes in thefirst diagonal set is labeled 618, and a spoke representative of thespokes in the second diagonal set is labeled 620. As shown, each setincludes a plurality of similarly positioned spokes arranged along thearc and extending between the upper and lower support members 612 and614. While the arc 610 may be constructed with all three sets of spokes,it may also be constructed with only one or two sets of spokes. In onepreferred embodiment, only the second set of diagonal spokes, asrepresented by 620, are provided. As discussed previously, the supportassembly 610 has a base portion or fixed end 622 that is attached to therest of the imaging apparatus of the present invention, with theremainder of the arc extending away from the base 622 to a free end. Insome embodiments, the remainder of the arc 610 is unsupported, andtherefore must be self-supporting. Because the arc 610 may be quiteheavy, it is necessary to construct it so as to resist sag and twistalong its length. By providing the spokes as represented by 620, somecompensation for sag of the arc may be provided. As will be clear tothose of skill in the art, by tightening the spokes 620, thecantilevered free end of the arc 610 may be raised relative to theposition it would take if the spokes are loosened. In some cases, thespokes as represented by 616 and 618 may also be provided to provideadditional structure or to provide for other compensation.

The arc 610 preferably includes a collimator that is constructed withmultiple sheets of plastic and lead. Experimentation has shown that ifthe upper support member 612 and lower support member 614 are not pulledtightly together, the end of the arc may be raised somewhat which causesthe individual sheets in the collimator assembly to slide very slightlyrelative to one another. If the two support members 612 and 614 are thencompressed against each other, the individual sheets of the collimatorassembly lock to one another and greatly increase the stiffness of theoverall arc 610. Therefore, the arc may be moved into a preferredposition, such as by using a fixture, and then various spokes may betightened to clamp the upper plate 612 and lower plate 614 tightly toone another.

Referring to FIG. 38, a cross-section of the arc is shown. In thisembodiment, the collimator assembly 630 is constructed with a pluralityof alternating layers of lead and plastic supported between a lowersupport plate 632 and an upper support plate 634. Threaded adjustingmembers 636 extend through the lower plate 614 to the lower supportmember 632 and through the upper plate 612 to the upper support member634. The various parts may not be to scale. By adjusting the adjustingmembers 636, the position of the collimator assembly 630 may be adjustedrelative to the remainder of the arc. As will be clear to those of skillin the art, the adjusting members are provided at a plurality oflocations along the length of the arcs, so that the position of thecollimator assembly may be adjusted throughout the length of the arc. Byadjusting the various adjusting members 636, the sag of the collimatorassembly along this length may be compensated for. In addition, anytwist in the collimator assembly may be adjusted out. Preferably, thecollimator assembly is adjusted such that each vane is substantiallyplanar.

FIG. 38 shows the aperture arc 638 positioned between the collimatorassembly and the patient side of the arc. The patient side of the arcmay have a thin sheet of radiolucent material, such as aluminum, asshown at 640. This thin piece of material 640 ties the upper supportmember 612 to the lower support member 614 so as to provide astructurally sound arc. A single spoke 642 is shown on the side of thearc away from the patient, and represents one of the spokes asillustrated at 616-620.

FIG. 39 shows an alternative embodiment where the thin piece of material640 is replaced by an additional spoke 644. The sheet of material 640 inFIG. 38 is preferred, as it provides a slight uniform reduction intransmission of photons, whereas the use of spokes causes a localizedreduction in the passage of photons. However, as will be clear to thoseof skill in the art, the machine may be calibrated such that thepresence of spoke 644 may be compensated for.

FIG. 40 illustrates an alternative embodiment where the sheet on thepatient side of the arc, here labeled 646, bends around the top of theupper support member 612 and around the bottom of the lower supportmember 614. This is a preferred construction as it provides easierattachment of the fasteners.

XVIII. Rebinning of Data

As will be clear to those of skill in the art, the imaging systemaccording to the present invention provides data in a form differentthan current imaging cameras, which have generally rectangular flatfaces. These traditional cameras take images from multiple angles, witheach image providing a two-dimensional “picture” of the patient fromthat angle. In the present invention, photons are received from multipleangles by the multiple detectors, with the lines of response for thevarious detectors being swept across the patient area as the aperturearc and detectors move relative to one another. Because the imaging isaccomplished in a different manner, it is preferred to initially processthe data from the present invention such that the data is put in theformat used with current machines. Then, traditional reconstructionsoftware may be used to process the resulting data. This interim step offormatting the data into the format currently provided by imagingdevices is referred to herein as “rebinning.”

In order to understand the rebinning of data in accordance with thepresent invention, it is best to first discuss how data is treated intraditional imaging devices. FIG. 41 illustrates a cross-section of apatient 650. A portion of an arc 652, which forms part of someembodiments of the present invention, is shown surrounding part of thecross-section of the patient 650. A traditional two-dimensional imagingcamera is shown schematically as 654 in a first position and again as656 at a position 90 degrees from the first position. A plurality ofparallel imaging lines is shown projecting from each camera 654 and 656.In processing data from the camera indicated at 654 and 656, theposition of incoming photons is generally designated using r and θ.These variables refer to the distance of the incoming photon from thecenter of the detector, r, and the angular position of the camera, θ.One of the incoming photon paths is labeled as 658 in FIG. 41. Thedistance of the path of this photon from the center of the camera inposition 654 is marked as r in the drawing. This data may be plotted ina graph referred to as the sinogram. A sample sinogram is shown in FIG.42. A sinogram plots the position from the center of the imager, r, withzero being in the center, versus the angular position of the camera, θ.If a point 660 in the very middle of the imaging area is the only pointemitting photons, a sinogram will be a straight vertical line as shownin FIG. 42. That is because the photons emitted from the single point660 will strike the center of the camera (r=0) at all angular positionsof a camera (θ=0 to 360 degrees). FIG. 43 illustrates a sinogram for asingle off-center point. As the imaging camera rotates about the imagingarea, the off center point appears to move side-to-side with respect tothe center of the camera, thereby giving a sine-shaped curve on thesinogram.

Referring again to FIG. 41, the cross-sectional slice illustrated as 650may be considered to be a single horizontal slice, if the patient ispositioned vertically. Obviously, a sinogram resulting from an actualpatient imaging session will be significantly more complex than shown inFIGS. 42 and 43. In addition, sinograms may be created for each “slice”of a patient. As discussed previously, the field of view in which thepatient is positioned may be defined as having a longitudinal axis. The“slice” of the patient will typically be perpendicular to thislongitudinal axis. This may also be referred to as a sensing plane. Aswill be clear to those of skill in the art, multiple sensing planes maybe defined so as to obtain different “slices” of the patient. Referringagain to FIG. 41, it will be appreciated that the distance r may bedefined as the distance in the sensing plane between a center point orcenter line of the camera and the position where the photon is received.In order to define the angular position of a traditional gamma camera, aposition line may be defined as extending perpendicularly between thelongitudinal axis and the center line of the camera in the sensingplane. The angle θ may then be defined as the angle between the positionline and an arbitrary base line also contained in the sensing plane andperpendicular to the longitudinal axis.

Referring back to FIG. 9B, it can be seen how the lines of response ofvarious detectors in the present device are swept across the field ofview as the aperture arc and the detectors move relative to one another.

FIG. 44 illustrates what may be considered a sinogram for an imagingsystem according to the present invention with an aperture arc havingsix apertures. If a single point is imaged, and the data is plotted on achart of aperture arc position versus the position on the detector arraywhere the photon is sensed, a plurality of angled parallel lines will becreated. This is because as the aperture arc position moves, theposition on the detector array where a photon from the point source canreach the detectors is also moved in a generally linear relationship. Itshould be noted that FIG. 44 is not to scale, but is intended only torepresent a general concept. In order to treat data such as shown inFIG. 44 using algorithms designed for data presented in the format ofFIGS. 42 and 43, the data must be resorted or rebinned.

Referring to FIG. 45, the slice 650 is shown with an emitting point 662.An aperture arc 663 with an aperture 664 surrounds the imaging area,with a detector arc or assembly 665, with a detector 666, surroundingthe aperture arc. A photon emitted from point 662 will pass through theaperture 664 and be received by the detector or sensor at 666. Becausethe position of the aperture 664 and the detecting position 666 areknown, the equivalent positional data, r and θ, for a two-dimensionalcamera may be computed. This equivalent r and θ is shown in FIG. 45 fora two-dimensional camera.

Referring now to schematic FIG. 46, a photon path is shown at 670, withan aperture arc shown at 672 and a detector assembly or arc shown at674. As shown, the photon path 670 will pass through an aperture at 676and strike a detector at 678. The center of the aperture arc is shown atC, with the equivalent r and θ for a two-dimensional imaging camerabeing indicated on the diagram. C also represents the center of thefield of view, and may lie on the longitudinal axis. The angle of theaperture 676 is expressed as φ, while the radius of the aperture arc isindicated as R_(app). The relationship between these variables is givenby the equation:

$\phi = {{\arcsin\left( \frac{r}{R_{app}} \right)} - \theta}$

FIG. 47 illustrates a similar relationship for the position of thedetector arc 678. In this case, the radius of the detector arc is givenas R_(det) and the angle of the position on the detector arc is given asΨ. The relationship between the variables is given by:

$\Psi = {{\arcsin\left( \frac{r}{R_{\det}} \right)} - \theta}$

As will be clear to those of skill in the art, sinograms may be createdfor each slice of the patient by plugging in values for θ and r for eachposition on the sinogram into the equations shown in FIGS. 46 and 47. Ineach case, this will provide an aperture position φ and a detectorposition Ψ. The intensity recorded for this combination of apertureposition and detector position may then be entered in the sinogram forthis combination of θ and r. This may be repeated until a completesinogram is created. Once a sinogram is created, the data may beprocessed as with traditional imaging devices. As will also be clear tothose of skill in the art, the rebinning may be accomplished using acomputing device.

XIX. Additional Calibration Configurations

As discussed with respect to FIG. 35, it is desirable to provide sometype of calibration source for calibrating an imaging system accordingto the present invention. In traditional two-dimensional imagingcameras, a flat sheet radioactive source is typically laid on top of acollimator, which is aimed upwardly. This calibration source exposes theentire surface of the camera to the same level of radioactive emission,such that the camera could be calibrated. That is, software adjustmentsmay be made if a certain portion of the camera reads high or low withrespect to other portions of the camera, such that after calibration,the resulting readout is uniform. These traditional calibrationapproaches have several drawbacks. First, the radioactive calibrationsource is large and heavy, and often difficult for technicians tomanipulate. They are typically stored in large lead boxes to avoidexcess radioactive exposure. However, due to the size and weight of thebox, the technician is often forced to leave the box in one location andthen carry the radioactive source, next to their body, into the roomwhere imaging is actually done. This is inconvenient and leads toadditional radioactive exposure by the technician. In addition, thecollimator positioned between the calibration source and the sensingsurface of the camera allows a passage of only about one in every tenthousand photons to reach the sensing surface. Therefore, thecalibration procedure is often time-consuming.

FIG. 35 presents one approach to calibrating the present invention.Additional approaches will be discussed hereinbelow. In each case, aradioactive source is provided in or adjacent an aperture of theaperture arc such that radiation is projected through the aperture arc,through the collimator, and to the detectors. Because of the design ofthe present invention, a much higher ratio of emitted photons reach thevarious detectors, allowing for reduced calibration time. After theradioactive calibration source is positioned, the aperture arc is movedand software may be used to calibrate the imaging system to adjust forthe variations in detector sensitivity and for the fact that photonsintercept the detectors at different angles depending on the position ofthe detectors relative to the aperture arc.

Referring now to FIG. 48, an alternative calibration source will bedescribed. A portion of an aperture arc is shown at 700 with a pair ofslot edge pieces 702 defining a slot 704. The calibration sourceincludes a generally tubular radioactive source 706 which is attached toa carrier including a lead shield 708 and a plastic carrier 710. FIGS.49-55 show the assembly and use of this calibration source. FIG. 49shows the plastic carrier 710. As shown, the carrier is arcuate with aconcave inner face. FIG. 50 shows the similarly shaped lead carriershield 708 nested in the plastic carrier 710. In some embodiments, thelead shield is approximately 2½ to 3 mm thick. Alternatively, the shieldmay be formed of tungsten, which allows it to be thinner for the sameamount of radioactive blocking. The radioactive source may be connectedto the carrier in a variety of ways. FIG. 51 shows the use of clips 712which may hold the source. Alternatively, it may be attached by gluingor by plastic coating the entire front side of the carrier to hold thesource in place. FIG. 52 shows the source itself 706 held by thecarrier. The source may be a tube filled with a radioactive materialsuch as cobalt particles embedded in an epoxy or other resin. FIG. 53shows the calibration source positioned in the aperture arc, with theside edge pieces removed for visibility. FIG. 54 shows the aperture arcwith the side pieces installed. FIG. 55 shows the radioactive sourcebeing slid into a pair of guide pieces to position it for calibration.The aperture arc may be moved side-to-side during the calibrationprocess. While the calibration source is shown as having a height tallerthan the aperture arc, it may also have a height equal to or shorterthan the height of the aperture arc.

As will be clear to those of skill in the art, the aperture arc ishoused inside the imaging arc such that access to the arc is not easilyprovided for positioning of the calibration source. A series of figuresstarting with FIG. 56 illustrates the assembly, positioning, and use ofa calibration source and holder or carrier for the source. FIG. 56 showsthe assembled holder 732. It includes a pair of side rails 720 withcurved upper ends positioned parallel to one another. The side rails 720are tied together at their top and bottom by interconnecting portions722 and 724. The bottom interconnecting portion 724 also acts as aspacer and may be said to be positioned on the inside of the side rails.A pair of inner rails 726 connect to the bottom spacer 724 and extendinggenerally parallel upwardly and spaced from the side rails 720. A pairof slots is defined between the inner rails 726 and the side rails 720.The open sides of the slots are closed by closure members 728. Supportflanges 730 are positioned adjacent the bottom of the assembled device.A front cross piece is shown at 734. In one embodiment, the cross piece734 includes a magnetic latch for locating the carrier or holder 732during use. FIG. 57 illustrates the carrier 732 in exploded view.

FIGS. 58 and 59 show a plastic block 736 that forms part of thecalibration source 746. As shown, the plastic block 736 has a recess inthe front face. A lead shield 738 is positioned in this recess in theplastic block 736. Guide pins 740 are provided extending from the sidesof the plastic block 736. A radioactive rod 742 is positioned againstthe inner face of the lead shield. A plastic or aluminum retainer 744 isthen positioned over the source to hold it against the lead shield andplastic block. This results in an assembled calibration source 746.

FIG. 60 shows the calibration source 746 positioned in the holder 732,with the holder supported by a hinge 748. FIG. 61 shows the holder 732and the source 746 rotated so as to see the back side of the plasticblock 736. FIG. 62 illustrates the holder 732 positioned between anupper support member 750 and lower support member 752 of the imagingarc, with an opening 754 defined through the upper member 750. FIG. 63adds the aperture arc 755 which has upper and lower guide rails 758 and760. FIG. 64 shows a portion of the upper support member 750 cut away.

FIG. 65 shows a detailed view of the hinge 748 at the bottom of thesource holder 746. In an alternative embodiment, a spring is provided,such as at the hinge 748, for spring biasing the upper end of the holdertowards the aperture arc 755.

Due to construction details of some embodiments of the presentinvention, the opening in the upper support member cannot be providedclose enough to the aperture arc such that the source can be droppedstraight into place. Instead, it needs to be positioned away from theportion of the member that supports the aperture arc. FIG. 66 shows thesource 746 being dropped through the opening 754 in the upper member 750into the holder 732. A portion of the member is cut away for visibility.FIG. 67 shows the side view of FIG. 66, and illustrates how the opening754 is positioned far enough back so that the carrier 746 does not moveinto position immediately adjacent the aperture arc as it extendsthrough the opening 754. Instead, it strikes the sloped back surface ofthe holder 732. This causes the source to tilt somewhat, which in turncauses the holder 732 to pivot back on the hinge so as to align with thesource. FIG. 68 shows the source 746 inserted fully into the holder 732with the holder moved back forwardly, with its motion being causedeither by the operator or by a spring in the hinge. The holder may thenbe held in place by the magnetic latch or by a spring, or by othermeans. FIGS. 69 and 70 illustrate additional views of this insertionprocess. FIG. 71 illustrates some sample dimensions for one embodimentof the holder 732.

FIG. 72 illustrates an alternative embodiment of a calibration sourcewhich may be used with a holder similar to that shown in the previousfigures. The calibration source is shaped similar to the source of FIG.48 and has an arcuate piece of lead or tungsten 798. The calibrationsource 800 has the piece of lead 798 wrapped in plastic 802 to protectthe lead and to stiffen it. A tube 804 of radioactive material isaffixed to the concave side of the lead and plastic assembly. Aparticular advantage to the calibration devices for use with the presentinvention is that they are much smaller, lighter, and easier to handlethan the previous calibration devices. FIG. 73 illustrates a calibrationholding box 810 with six calibration sources positioned inside of it.Each source may nest such that they fit compactly into the box 810. Thetotal weight of the box and calibration sources may be as little as 6 or10 pounds, making it easy for a technician to handle without unnecessaryexposure.

XX. Curved Crystal Detectors

As discussed previously, the present invention may be made with eithersolid state detectors, such as CZT, or with scintillation materialbacked by photo tubes. In earlier embodiments, individual pieces ofscintillation material were assembled side-by-side. According to apreferred alternative, a single large curved crystal of scintillationmaterial may be provided, such as shown in FIG. 74 at 820. This singlecurved piece of crystal may be scored along its face, with the scarelines being cut partially through the crystal so as to divide it intodifferent photon receiving regions. FIG. 75 illustrates the crystal 820backed by a plurality of photo tubes 822.

XXI. Brain Imaging

As will be clear to those of skill in the art, imaging systems inaccordance with the teachings of the present invention may be used toimage a variety of items, including body parts other than the chest.FIG. 76 shows an embodiment of the present invention configured forbrain imaging. While such a device may be constructed similarly to priorillustrated versions, a preferred embodiment is formed as a completecircle rather than an arc that extends between a pair of spaced-apartends. The device 900 in FIG. 76 includes a base 902 that supports apatient and also supports an imaging ring 904. As shown, the imagingring 904 surrounds the patient's head such that the patient's brain maybe imaged. The ring may be larger or smaller than shown, for example toimage other objects such as a patient's chest or body. The imaging ring904 may be constructed substantially identically to earlier embodimentsof the present invention with the various components being formed asrings rather than arcs. Such a device includes an aperture ring, a ringof collimating vanes and a sensor or detector ring. The aperture ring orthe sensor or detector ring moves such that lines of response are sweptacross the field of view, preferably including the area containing thepatient's brain. It should be noted that while the illustratedembodiment positions the patient in an upright position, otherembodiments may be adjustable and/or provide a more horizontal positionwith the imaging ring disposed in a more vertical orientation.

XXI. Embodiment with Enhanced Resolution

As will be clear to those of skill in the art, imaging resolution isparticularly important when imaging a patient's brain. As discussedpreviously, SPECT-based imaging systems generally face a tradeoffbetween imaging efficiency or sensitivity and imaging resolution. Someembodiments of the present invention improve on this traditionaltradeoff. That is, they can provide substantially enhanced resolutionand/or sensitivity as compared to traditional systems. The presentinvention may also be modified to further enhance resolution while stillmaintaining acceptable levels of sensitivity and imaging speed.Embodiments of the present invention modified to provide enhancedresolution are particularly applicable to brain imaging. Prior discussedembodiments of the present invention may also be modified to enhanceresolution for imaging other body parts, such as the heart, as well asfor other applications, as will be discussed further hereinbelow.

FIG. 77 provides a partially cutaway perspective view of an embodimentof a brain imaging ring with enhanced imaging resolution. As withearlier embodiments, the imaging ring preferably is housed in a case906, which is here shown in phantom lines. The case 906 has a generallyring-like shape and defines a central generally cylindrical opening 908,part or all of which defines the field of view of the imaging ring 904.Moving outwardly from the field of view, an aperture ring 910 isprovided with one or more apertures 912 defined therethrough. Theaperture ring 910 may also be referred to as a photon blocking member.In the illustrated embodiment, the aperture ring 910 includes aplurality of apertures 912 defined therethrough. Each of the apertures912 preferably allows the passage of substantially all photons alignedtherewith while blocking all or a substantial percentage of the photonsthat strike the aperture ring 910 in other locations. The aperture ring910 is preferably constructed similarly to earlier discussed embodimentsof an aperture arc. For example, the ring 910 may be constructed of leador a similar high attenuation material and have a thickness (typicallyapproximately 3 mm) so as to affect essentially complete absorption ofphotons emitted by the patient. The apertures 912 may have any ofseveral shapes, sizes and configurations, such as discussed previously.In the illustrated embodiment, if the imaging ring 904 is considered tobe generally horizontal, the apertures 912 are each generally verticalslots or openings defined through the ring 910. Put another way, theimaging ring 904 may be said to generally define an imaging plane, alongitudinal axis may be considered to extend through the center of theopening 908 and be generally perpendicular to the plane, and theapertures 912 in the illustrated embodiment may be said to also beperpendicular to the imaging plane. In other embodiments, the apertures912 are angled with respect to the longitudinal axis. Preferably, theaperture ring 910 is rotated so as to sweep lines of response across thefield of view. Rotation of the aperture ring may be achieved in avariety of ways, such as by using a motor 914.

Referring again to the illustrated embodiment of FIG. 77, a collimatorassembly 916 is provided outboard of the aperture ring 910 and generallycoaxial therewith. Alternatively, the collimator assembly may be inboardof the aperture ring. As with earlier embodiments of a collimatingassembly, the assembly 916 consists of a plurality of generally parallelvanes 918 that are spaced apart so as to allow the passage of photonstherebetween. Together, the aperture ring 910 and the collimatingassembly 916 cooperate to limit which photons reach a sensor or detectorassembly outboard of the aperture ring and collimating assembly.Preferably the vanes 918 are formed of a photon attenuating materialsimilar to the aperture ring 910 and the number, size and thickness ofthe vanes may be varied depending on the application. Values for thenumber, size, shape, and dimensions (such as thickness and depth) of thevanes may be equal to any of the values discussed previously herein ormay have other values depending on the application. The collimatingassembly 916 may be formed in accordance with previously discussedapproaches, such as by stacking alternating sheets of photon attenuatingmaterial and radiolucent material. In the illustrated embodiment, thevanes 918 are each generally planar ring-shaped members that aregenerally parallel to the imaging plane and perpendicular to thelongitudinal axis. Alternatively, the vanes may be disposed at otherangles, such as those discussed previously with respect to priorembodiments of the present invention.

Referring again to the embodiment of FIG. 77, a mask or mask ring 920 isprovided outboard of the collimating assembly 916 and aperture ring 910,and coaxial therewith. The mask ring 920 is an element not present inprior embodiments of the present invention. The mask ring 920 is similarto the aperture ring 910 in that it is formed of a photon-attenuatingmaterial such as lead and has a plurality of apertures 922 definedtherethrough. The mask ring 920 is preferably formed of a sufficientthickness of photon-attenuating material that substantially all photonsnot aligned with one of the apertures 922 are blocked or absorbed. Aswill be discussed further hereinbelow, the provision of the mask ringmay increase the resolution of the overall imaging system above theintrinsic resolution of the detector or sensor assembly. This is due tothe fact that the mask ring 920 may more precisely determine theposition of a photon actually reaching the sensor or detector assemblysince it is known that any photon that actually reaches the detector orsensor assembly must have been aligned with one of the apertures 922.Like the aperture ring 910, the mask ring 920 is preferably movedrelative to the remainder of the device so as to move the apertures andthereby expose different portions of a sensor or detector assembly tophotons passing therethrough. This movement may be power assisted, suchas by using motor 924. The mask ring 920 is an optional feature of thepresent invention, and may be eliminated from any of the embodimentsdiscussed herein.

Referring again to the embodiment of FIG. 77, a detector or sensorassembly 930 is provided outboard of the mask ring 920 and generallycoaxial therewith. As an alternative to rotating the aperture ring 910,the detector or sensor assembly may be moved relative to the field ofview, thereby sweeping lines of response across the field of view.Movement of the detector or sensor assembly may also be used in place ofmovement of the mask. In the illustrated embodiment the detectorassembly 930 consists of a ring of scintillation material 932 surroundedby a plurality of photomultiplier tubes 934 or other photo-detectorssuch as solid state photodiodes. Alternatively, other types of detectoror sensor assemblies may be used to provide the detector ring 930. Thismay include solid state or scintillation-based sensors and may includeone-dimensional or two-dimensional sensors such as shown in FIGS. 22-24and sensors such as shown in FIGS. 28-32. The photo-detectors orphotomultiplier tubes 934 illustrated are smaller than typically units.Alternatively, standard sized tubes, having a diameter of two to threeinches, may be used. In such an embodiment, fewer tubes would fit aroundthe perimeter of the device, such as one half to one third as manytubes. There may also be fewer tubes along the longitudinal direction.

FIG. 78 provides a cross-sectional view of the imaging ring 904 of FIG.77, taken along lines A-A, though the photomultiplier tubes are notillustrated. FIG. 79 provides a generally horizontal cross section ofthe imaging ring of FIG. 77, taken along lines B-B. FIG. 79 additionallyincludes fines of response D and E, representing photons travelingoutwardly from the field of view. An object with a brain shaped outlineis also shown in the field of view. Preferably, the imaging ring issized such that a person's head fits inside, and the brain fills themajority of the field of view. The brain may have a different size,shape or orientation than shown. In one embodiment of the presentinvention, the opening in the center of the imaging ring has a radius of14 to 16 centimeters so as to receive a normal size adult head. It ispreferred that the imaging ring have a longitudinal dimension sufficientto image the brain. As such, an embodiment of the ring is sized toprovide an image height of 15 to 18 centimeters. The image height islimited by the height of the apertures and the sensors. Therefore, theoverall device may have a longer longitudinal dimension to achieve theseimage heights.

Referring again to FIG. 79, both lines of response D and E arepositioned such that they pass through apertures 912 in the aperturering 910 and are assumed to be generally horizontal such that theyextend between two vanes 918 of the collimating assembly 916. However,only line of response D is illustrated as reaching the detector orsensor ring 930 due to its alignment with one of the apertures 922 inthe mask ring 920. Line of response E, on the other hand, is not alignedwith an aperture 922 in the mask ring 920 and therefore does not reachthe detector or sensor ring 930. As will be clear to those of skill inthe art, the inclusion of the mask ring 920 reduces the overallsensitivity of the imaging system since not all photons passing throughapertures 912 in the aperture arc and between vanes 918 of thecollimator assembly 916 actually reach the sensor or detector ring 930.In the illustrated embodiment, the mask ring 930 allows about one-thirdof the photons reaching it to pass through one of the apertures.

Referring now to FIG. 80, a portion of an imaging ring or arc is showngenerally at 940. The imaging ring or arc 940 includes an aperture ringor arc 942 with at least one aperture 944 therethrough, a plurality ofcollimating vanes 950 and a sensor or detector ring or arc 946. FIG. 80also illustrates an object 141 being imaged by the imaging ring or arc940. Two lines of response G and F are shown extending from the object941 through the aperture 944 to the sensor or detector assembly 946. Thelines of response G and F originate at two closely spaced points G′ andF′ within the object 941. As will be clear to those of skill in the art,if two points, such as G′ and F′ are very close to one another, theirlines of response G and F may strike the sensor or detector 946 atlocations that are also very close to one another, such as illustratedat G″ and F″. If the distance between points G″ and F″ are less than theintrinsic resolution of the detector or sensor assembly 946, the imagingsystem may be incapable of distinguishing between the two points andtherefore may not be able to distinguish between the points of origin G′and F′ in the object 941. It should be noted that the distance betweenG′ and F′ has been exaggerated in FIG. 80 to demonstrate this situationand that the illustrated distances may not reflect the actual resolutionof a device according to the present invention. Also, while the lines ofresponse G and F are illustrated as parallel to one another, they may beat other angles and still lead to a situation where the points of impactG″ and F″ are not sufficiently separated to allow the system to resolvethem as two separate points. The resolution of the overall system may beimproved by reducing the size of the aperture 944, at a cost of reducedsensitivity, or by improving the intrinsic resolution of the sensor ordetector system 946. However, it may not be possible or practical toincrease the intrinsic resolution of the detector or sensor system 946.

Referring now to FIG. 81, a portion of an imaging system 960 isillustrated, which is similar to the system 940 of FIG. 80, but furtherincludes a mask ring or arc 962 with apertures 964 defined therethrough.As with the system 940, the system 960 includes an aperture ring or arc966 with apertures 968 defined therethrough, a plurality of collimatorvanes 970 and a detector or sensor assembly 972. The same lines ofresponse G and F are shown extending from points G′ and F′ within object941. However, in this case line of response F is blocked by the maskring or arc 962 such that it strikes the arc or ring next to an aperture964. Line of response G is aligned with an aperture 964 in the mask ringor arc 962 and therefore strikes the detector or sensor assembly 972 atpoint G″. Because the position of the aperture 964 in the mask ring orarc 962 is known, the system can therefore more precisely determine theposition of point G″, since any photon striking the detector or sensorassembly 972 must have passed through one of the apertures 964. In onepreferred embodiment, the apertures 964 are sized so as to have a widthapproximately one-third of the intrinsic resolution of the detector orsensor assembly 972, and the apertures 964 are spaced apart by adistance equal to approximately two-thirds of the intrinsic resolution.As such, the resultant resolution of the overall system can beapproximately three times better than without the mask ring or arc 962.While the sensitivity and imaging speed of the overall system is reducedby the mask ring or arc, the tradeoff is warranted where increasedresolution is beneficial. In addition, some embodiments of the presentinvention have sufficient sensitivity and imaging speed that thereduction imposed by the mask ring or arc is not particularlydetrimental. The ratio of open space and the width of the apertures maybe decreased so as to increase the resolution of the imaging device,though this comes at the cost of increased imaging time, or may beincreased so as to reduce imaging time. As such, the size of aperturemay depend on the application. As with earlier embodiments, theapertures may be adjustable. As one alternative, the mask may include apair of rings, each with apertures, and the rings can be aligned so asto provide large apertures or may be rotated relative to each other soas to progressively reduce the aperture size. This may be adjusted onetime and set, or may be adjustable in the field depending on theapplication of the machine. Adjustable apertures may also be provided inaccordance with earlier embodiments of the aperture arc. As a furtheralternative, the mask ring may be removable and/or replaceable to adjustthe resolution/imaging time tradeoff. As will be clear to those of skillin the art, it is preferred that the position of the apertures in themask be known very accurately and precisely. For example, it ispreferred that the position be known to a level of accuracy greater thanthe intrinsic resolution of the sensors. In some embodiments, thepositional accuracy of the mask is less than one pixel. As will also beclear to those of skill in the art, the mask position may be calibratedonce a device is assembled, and periodic calibration may be helpful.

Referring now to FIG. 82, the system 960 is again illustrated, but withthe mask ring or arc 962 moved slightly counterclockwise as indicated byarrow H. The aperture ring or arc 966 is illustrated in the sameposition as in FIG. 81. With the mask arc or ring 962 in the newposition, line of response G is now blocked by the mask ring or arc 962while line of response F passes through the aperture 964 resulting indetection at F″.

As with prior described embodiments of the present invention, it ispreferred that the aperture arc or ring 966 be moved relative to thedetector or sensor assembly 972 such that lines of response are sweptacross the field of view. FIG. 83 illustrates the system 960 with theaperture arc or ring 966 rotated a short distance counterclockwise suchthat the aperture 968 is moved downwardly. A different line of responseK now passes through the aperture 968, is aligned with aperture 964 inthe mask ring or arc 962 and results in imaging at K″.

As will be clear to those of skill in the art, the aperture ring or arc966 and the mask ring or arc 962 may be moved in a variety of ways so asto achieve high resolution imaging of the entire field of view. In oneapproach, the mask ring or arc 962 is positioned in a first position andthe aperture arc or ring 966 is moved such that lines of response areswept across the entire field of view. The mask ring or arc 962 may bethen moved to a second position and the movement of the aperture ring orarc 966 is repeated. The process can then be again repeated until acomplete high resolution image is formed. In the version of the presentinvention in which the mask ring or arc has apertures coveringapproximately one-third of its surface, the process will typically berepeated for three positions of the mask ring or arc, though otherstrategies may be employed. In a second approach, the mask ring or arcis moved while the aperture ring or arc is held stationary. In a furtherapproach, both are moved at the same time in a pattern that allowscomplete high resolution imaging.

While the embodiment illustrated in FIG. 77 is illustrated as forming animaging ring, the approach of using a mask may also be applied toarcuate versions of the present invention. Also, while FIGS. 77-83illustrate arc or ring-shaped versions of the present invention, thestrategy of using a mask in combination with collimating vanes and anaperture arc may also be applied to planar image devices, such as shownin FIG. 84. In FIG. 84, a flat or generally flat detector assembly isshown at 980 and a field of view is shown at 982. A generally flat maskplate 984 is positioned adjacent the detector assembly 980 and hasapertures 986 defined therethrough. A plurality of collimating vanes 988and an aperture plate 990 are positioned between the detector assembly980 and the field of view 982. The aperture plate 990 has a plurality ofapertures 992 defined therethrough. The operation of the embodiment ofFIG. 84 would be similar to that described with respect to FIGS. 81-83.

Referring now to FIG. 85, an arc version of an imaging system 1000 isillustrated. This embodiment is similar to the embodiment of FIG. 77,but forms a 180-degree arc. Arcs extending more or less than 180 degreesmay also be constructed, with a dimension somewhat larger than 180degrees being one preferred version. This may be provided with orwithout a mask. An arc shaped imager is particularly useful for imagingbody parts or organs that are positioned in the body asymmetrically,such as the heart, kidneys or spine.

Referring now to FIG. 86, another alternative embodiment of an imagingring is shown generally at 1050. As mentioned previously, the aperturesin the aperture ring do not necessarily have to be parallel to thelongitudinal axis of the opening through the ring and the collimatorvanes do not necessarily have to be perpendicular to the longitudinalaxis. In the embodiment illustrated in FIG. 86, the aperture ring 1060has a plurality of apertures 1062 which are generally each perpendicularto the longitudinal axis. Such an aperture ring 1060 may be constructedin a variety of ways, such as by stacking photon-attenuating materialwith radiolucent material, or by interconnecting adjacent pieces ofphoton-attenuating material with a reinforcing member such as shown at1064. In this embodiment, the collimating assembly 1066 consists of aplurality of radial vanes 1068 which are each generally disposed in aplane perpendicular to the imaging plane. As will be clear to those ofskill in the art, it is preferred that the apertures 1062 in theaperture ring 1060 and the vanes 1068 be at angles to one another suchthat they cooperate to collimate photons passing therethrough. In someembodiments, the vanes 1068 are thinner and more closely spaced thanshown. A thickness of about 1 mm and a spacing, at the inside end, ofabout 3 mm may be used in some embodiments. FIG. 86 also illustrates amask ring 1070 constructed similar to the mask ring of FIG. 77.Alternatively, the mask ring may have apertures that are angleddifferently than shown, and may even be parallel to the imaging plane,such as apertures 1062 in the aperture ring 1060. As will be clear tothose of skill in the art, the aperture ring 1060 is moved along thelongitudinal axis so as to sweep lines of response across the field ofview. The mask ring 1070 is rotated about the longitudinal axis so as toexpose different parts of the detector assembly 1072. If a mask ringwere provided with apertures generally parallel to the imaging plane,the mask would be moved along the longitudinal axis so as to expose allparts of the detector assembly 1072. One preferred embodiment of thepresent invention is arc shaped, rather than ring shaped, and has anaperture arc and a mask arc both with apertures generally parallel tothe imaging plane. This is beneficial because the aperture arc and maskarc may both be moved longitudinally, rather than rotated. This preventsthe aperture arc and mask arc from moving beyond the ends of the arc ofsensor and intruding into any gap between the spaced apart ends of thearc. This eliminates the need for flaps such as were illustratedearlier. Another preferred embodiment of the present invention is ringshaped and design for brain imaging, and has angled collimator fins andapertures, such as were shown in the embodiment of FIG. 12. This isbeneficial because it provides similar resolution both parallel to theimaging plane and perpendicular thereto. For example, the collimatorfins and apertures may both be angled at approximately 45 degrees to thelongitudinal axis.

Other variations on the disclosed preferred embodiments will be clear tothose of skill in the art. It is the following claims, including allequivalents, that define the scope of the present invention.

1. A single photon emission computed tomography system for producing multiple tomographic images of the type representing a three-dimensional distribution of a photon-emitting radioisotope in a field of view, said system comprising: a detector assembly adjacent the field of view, said detector assembly including a photon-responsive detector is operable to detect if a photon strikes the detector, the detector assembly operable to detect photons emitted from the field of view; a collimating assembly including at least two spaced apart collimating vanes of photon-attenuating material, each of the vanes being disposed between the detector assembly and the field of view; a photon-blocking member disposed between the field of view and the detector, the blocking member having an aperture defined therethrough for passage of photons aligned with the aperture, a line of response being defined from the detector assembly through the aperture; a mask disposed adjacent the detector assembly and between the detector assembly and the field of view, the mask having at least one aperture defined therethrough for passage of photons aligned with the aperture; and a displacement actuator for moving the photon-blocking member relative to the detector assembly such that the aperture is displaced relative to the detector assembly and the line of response is swept across at least a portion of the field of view.
 2. The system according to claim 1, wherein a longitudinal axis is defined thin the field of view, the photon blocking member being a ring of photon attenuating material generally coaxial with the longitudinal axis, the photon blocking member generally disposed in an imaging plane, the aperture defined in the photon blocking member comprising a plurality of spaced apart aperture slots defined thru the ring of photon attenuating material.
 3. The system according to claim 2, wherein each aperture slot is generally parallel to the longitudinal axis and perpendicular to the imaging plane.
 4. The system according to claim 2, wherein the photon blocking member is disposed between the collimating assembly and the field of view.
 5. The system according to claim 1, wherein a longitudinal axis is defined thru the field of view, the mask being a ring of photon attenuating material generally coaxial with the longitudinal axis, the mask generally disposed in an imaging plane, the aperture defined in the mask comprising a plurality of spaced apart aperture slots defined thru the ring of photon attenuating material.
 6. The system according to claim 5, wherein each aperture slot is generally parallel to the longitudinal axis and perpendicular to the imaging plane.
 7. The system according to claim 1, wherein a longitudinal axis is defined thru the field of view, the collimating vanes each being generally ring shaped and disposed in a plane perpendicular to the longitudinal axis, each vane having a depth extending radially from a front edge to a rear edge in the plane and a thickness perpendicular to the plane, the depth of each vane being greater than the thickness.
 8. The system according to claim 7, wherein a radiolucent material is disposed in the spaces between said vanes.
 9. The system according to claim 7, wherein the collimating assembly includes only vanes that are generally parallel to each other and generally perpendicular to the longitudinal axis.
 10. The system according to claim 1, wherein a longitudinal axis is defined thru the field of view, the detector assembly being generally ring shaped and generally coaxial with the longitudinal axis, the detector assembly generally disposed in an imaging plane.
 11. The system according to claim 10, wherein the detector assembly comprises a ring of scintillation material with an outwardly adjacent ring of photo detectors.
 12. The system according to claim 10, wherein the detector assembly comprises a plurality of solid state photon detectors disposed generally in a ring.
 13. The system according to claim 1, wherein the detector assembly, the collimating assembly, the photon-blocking member and the mask are each generally arcuate members extending between a pair of spaced apart ends.
 14. The system according to claim 1, wherein each arcuate member extends over an arc between 180 and 360 degrees.
 15. The system according to claim 1, wherein the photon-blocking member and the mask are each generally flat members.
 16. The system according to claim 1, wherein a generally vertical longitudinal axis is defined thru the field of view, the system being generally ring shaped and generally coaxial with the longitudinal axis, the system disposed in a generally horizontal imaging plane.
 17. The system according to claim 1, wherein the collimating assembly comprises a stack of alternating sheets of photon-blocking material and sheets of radiolucent material, the sheets of photon-blocking material defining the collimating vanes.
 18. The system according to claim 1, further comprising a chair-like base having a generally horizontal bottom portion for supporting a patient's hips and a generally vertical back portion of supporting the patient's back.
 19. The system according to claim 18, wherein the detector assembly, the collimating assembly, the photon-blocking member and the mask are each generally ring shaped members and are disposed such that the field of view is positioned to receive a patient's head when the patient is seated in the chair-like base.
 20. The system according to claim 1, wherein the mask is moveable relative to the detector so as to expose different portions of the photon receiving face of the detector to photons passing through the mask. 